qemu: driver: Allow passing disk target as top image with block commit

qemu: driver: Allow passing disk target as top image with block commit

Since we allow active layer block commit the users are allowed to commit
the top of the chain (e.g. vda) into the backing image. The API would
not accept that parameter, as it tried to look up the image in the
backing chain.

Add the ability to use the top level image target name explicitly as the
top image of the block commit operation.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1451394