From R73-11366.0.0, all boards failed to start VM

https://stainless.corp.google.com/search?exclude_cts=true&exclude_non_release=true&test=%5Etast%5C.vm%5C.&suite=%5Ecrosbolt%5C_perf%5C_perbuild%24&view=matrix&col=build&row=test&first_date=2018-11-27&last_date=2018-12-10

https://stainless.corp.google.com/search?exclude_retried=true&first_date=2018-12-04&master_builder_name=&builder_name_number=&shard=&exclude_acts=true&builder_name=&master_builder_name_number=&owner=&retry=&exclude_cts=true&exclude_non_production=false&hostname=&board=&branch=&test=%5Etast%5C.vm%5C.CrostiniStartTime%24&suite=%5Ecrosbolt%5C_perf%5C_perbuild%24&build=%5ER73%5C-11366%5C.0%5C.0%24&status=FAIL&status=ERROR&status=ABORT&reason=&waterfall=&exclude_not_run=false&last_date=2018-12-10&exclude_non_release=true&exclude_au=true&model=&view=list

Error message:
Failed to start VM: failed to start VM: Failed to mount shared directory


2018-12-08T20:00:20.143908+00:00 INFO vm_concierge[13745]: Mounting 9P file system from port 16384 on /mnt/shared
2018-12-08T20:00:20.145054+00:00 NOTICE kernel: [12533.789867] audit: type=1326 audit(1544299220.143:1600): auid=4294967295 uid=1000 gid=1000 ses=4294967295 subj=u:r:minijail:s0 pid=13779 comm="9s" exe="/usr/bin/9s" sig=31 arch=c000003e syscall=44 compat=0 ip=0x7a2211600fbd code=0x0
2018-12-08T20:00:20.156066+00:00 INFO crash_reporter[13822]: libminijail[13822]: mount '/dev/log' -> '/dev/log' type '' flags 0x1001
2018-12-08T20:00:20.162125+00:00 INFO crash_reporter[13823]: libminijail[17]: mount '/dev/log' -> '/dev/log' type '' flags 0x1001
2018-12-08T20:00:20.170411+00:00 WARNING crash_reporter[13822]: [user] Received crash notification for 9s[13779] sig 31, user 1000 group 1000 (developer build - not testing - always dumping)
2018-12-08T20:00:20.172349+00:00 INFO crash_reporter[13822]: State of crashed process [13779]: S (sleeping)
2018-12-08T20:00:20.172808+00:00 INFO crash_reporter[13822]: Accessing crash dir '/var/spool/crash' via symlinked handle '/proc/self/fd/5'
2018-12-08T20:00:20.173695+00:00 INFO crash_reporter[13823]: Processing selinux violation: always collect from developer builds
2018-12-08T20:00:20.174236+00:00 INFO crash_reporter[13823]: Accessing crash dir '/var/spool/crash' via symlinked handle '/proc/self/fd/8'
2018-12-08T20:00:20.195594+00:00 INFO crash_reporter[13822]: Stored minidump to /var/spool/crash/9s.20181208.120020.13779.dmp
2018-12-08T20:00:20.195996+00:00 INFO crash_reporter[13822]: Leaving core file at /proc/self/fd/5/9s.20181208.120020.13779.core due to developer image
2018-12-08T20:00:20.202871+00:00 INFO seneschal[13747]: Process 2 killed by signal 31 (core dumped)
2018-12-08T20:00:20.204191+00:00 ERR vm_concierge[13745]: Failed to mount 9P server on /mnt/shared inside VM 3: failed to mount file system
2018-12-08T20:00:20.204229+00:00 ERR vm_concierge[13745]: Failed to mount 
2018-12-08T20:00:20.256803+00:00 INFO VM(3)[13788]: [    1.682983] Unregister pv shared memory for cpu 0#015
2018-12-08T20:00:20.261468+00:00 INFO VM(3)[13788]: [    1.690516] Unregister pv shared memory for cpu 1#015
2018-12-08T20:00:20.265669+00:00 INFO VM(3)[13788]: [    1.695455] reboot: Restarting system#015
2018-12-08T20:00:20.270864+00:00 INFO VM(3)[13788]: [    1.698999] reboot: machine restart#015
2018-12-08T20:00:20.275635+00:00 INFO localhos[13787]: crosvm[4]: [src/linux.rs:910] vcpu requested shutdown
2018-12-08T20:00:20.316080+00:00 INFO localhos[13787]: crosvm[4]: [src/main.rs:523] crosvm has exited normally
2018-12-08T20:00:20.338737+00:00 INFO vm_concierge[13745]: process 5 did not exit from signal 9 in 0 seconds
2018-12-08T20:00:20.339433+00:00 INFO seneschal[13747]: Received request to stop server
2018-12-08T20:00:20.349828+00:00 INFO vm_concierge[13745]:  Process 5 exited with status 0

Hi Dylan, could you help triage this issue?