GCEs in zone us-east1-a cannot support KVM because 

"
us-east1-a has Ivy Bridge machines, so the VMs in that zone would be
configured by default as Ivy Bridge, and we only support nesting on
Haswell or later. You should be able to get nesting to work in
us-east1-a if you use the CPU selection feature to select Skylake.
"

Now the option of specifying CPU platform is only supported in "gcloud beta", so plan to migrate all GCEs for tryservers to the central zone.