qemu: Use error from Finish instead of "unexpectedly failed"

qemu: Use error from Finish instead of "unexpectedly failed"

When QEMU exits on destination during migration, the source reports
either success (if the failure happened at the very end) or unhelpful
"unexpectedly failed" error message. However, the Finish API called on
the destination may report a real error so let's use it instead of the
generic one.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>