Merge changes from topic "liblp_virtualab_prepare"

* changes:
  liblp: Don't halve super partition for virtual a/b devices
  liblp: MetadataBuilder::GetBlockDevicePartitionName
  liblp_test_static: GetMockedInstance->GetMockedPropertyFetcher
  libsnapshot: UnmapSnapshot: don't delete device that doesn't exist.
  liblp: change some functions to use string_view