fastboot: Add pvmfw

The pvmfw partition holds the protected VM firmware that is used by
Protected KVM to bootstrap the trust in protected VMs. Teach fastboot
about the partition so it gets flashed with flashall.

pvmfw is an AVB chained partition so is marked as BootCritical to allow
ABL's AVB verification to pass.

Test: tab complete and flashall
Bug: 171280178
Change-Id: Ie4cc478de25a945bc510488d87c2bee3aa5031f2