qemu: Abort failed post-copy when we haven't called Finish yet

qemu: Abort failed post-copy when we haven't called Finish yet

When migration fails after it already switched to post-copy phase on the
source, but early enough that we haven't called Finish on the
destination yet, we know the vCPUs were not started on the destination
and the source host still has a complete state of the domain. Thus we
can just ignore the fact post-copy phase started and normally abort the
migration and resume vCPUs on the source.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>