vmm_test: support ubuntu images that have shim to not require a reboot when a tpm is enabled

[chris-oo]

For some reason, shim wants an explicit boot entry in UEFI if a tpm is present. If it's not present, it writes one, then issues a reboot. This breaks certain tests as the test infra does not expect a reboot, which we can hack around for now.

Instead, we should figure out how to create a UEFI relative path boot entry and have petri set that up. This way, we don't need these hacks to support multi reboots because shim wants this behavior.