Because SharedHostDir.cleanup is racy:

        # It's possible that the directory is no longer mounted (e.g. if the        
        # system was rebooted), so check before unmounting.                         
        utils.run('if findmnt "%(path)s" > /dev/null;'                              
                  '  then sudo umount "%(path)s";'                                  
                  'fi' %                                                            
                  {'path': self.path})                                              
        utils.run('sudo rm -r "%s"' % self.path)


We can't be sure that the umount completes before the 'sudo rm -r'
Sometimes, the unmount takes a while leading to unittest (and real) failures:

autotest-0.0.2-r8709: 
autotest-0.0.2-r8709: ======================================================================
autotest-0.0.2-r8709: ERROR: testHostDirCreationAndCleanup (autotest_lib.site_utils.lxc.shared_host_dir_unittest.SharedHostDirTests)
autotest-0.0.2-r8709: Verifies that the host dir is properly created and cleaned up when
autotest-0.0.2-r8709: ----------------------------------------------------------------------
autotest-0.0.2-r8709: Traceback (most recent call last):
autotest-0.0.2-r8709:   File "/build/guado_moblab/tmp/portage/chromeos-base/autotest-0.0.2-r8709/work/autotest-0.0.2/site_utils/lxc/shared_host_dir_unittest.py", line 45, in testHostDirCreationAndCleanup
autotest-0.0.2-r8709:     host_dir.cleanup()
autotest-0.0.2-r8709:   File "/build/guado_moblab/tmp/portage/chromeos-base/autotest-0.0.2-r8709/work/autotest-0.0.2/site_utils/lxc/shared_host_dir.py", line 69, in cleanup
autotest-0.0.2-r8709:     utils.run('sudo rm -r "%s"' % self.path)
autotest-0.0.2-r8709:   File "/build/guado_moblab/tmp/portage/chromeos-base/autotest-0.0.2-r8709/work/autotest-0.0.2/client/common_lib/utils.py", line 738, in run
autotest-0.0.2-r8709:     "Command returned non-zero exit status")
autotest-0.0.2-r8709: CmdError: Command <sudo rm -r "/build/guado_moblab/tmp/portage/chromeos-base/autotest-0.0.2-r8709/temp/tmpM5YRyX/host"> failed, rc=1, Command returned non-zero exit status
autotest-0.0.2-r8709: * Command: 
autotest-0.0.2-r8709:     sudo rm -r "/build/guado_moblab/tmp/portage/chromeos-
autotest-0.0.2-r8709:     base/autotest-0.0.2-r8709/temp/tmpM5YRyX/host"
autotest-0.0.2-r8709: Exit status: 1
autotest-0.0.2-r8709: Duration: 0.0058171749115

See the autotest unittest failure here:
https://uberchromegw.corp.google.com/i/chromiumos.tryserver/builders/paladin/builds/3959