Allow disabling AVB on special vbmeta partitions

The vbmeta partition might have additional prefix
(e.g., guest_vbmeta_a) on some virtual machine
environment. Allowing rewriting the disable-verity/
disable-verification bits in the vbmeta image upon
flash under such use case.

Bug: https://partnerissuetracker.corp.google.com/u/1/issues/181791553
Test: manual tests with different names of vbmeta partition
Change-Id: Iacadaf587fce1e1e7fef946f56bbfc7f4e8233d8