qemu: add disk post parse to qemublocktest

qemu: add disk post parse to qemublocktest

The post parse callback is part of the real (non-test) processing flow.
This commit adds it (for disks) to the qemublocktest flow as well.
Specifically, this will be needed for tests that use luks encryption,
so that the default encryption engine (which is added in an upcoming commit)
will be overridden by qemu.

Signed-off-by: Or Ozeri <oro@il.ibm.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>