qemu: Properly report failed migration

qemu: Properly report failed migration

Because we are polling we may detect some errors after we asked QEMU for
migration status even though they occurred before. If this happens and
QEMU reports migration completed successfully, we would happily report
the migration succeeded even though we should have cancelled it because
of the other error.

In practise it is not a big issue now but it will become a much bigger
issue once the check for storage migration status is moved inside the
loop in qemuMigrationWaitForCompletion.

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