migration: multifd_save_cleanup() can't fail, simplify

migration: multifd_save_cleanup() can't fail, simplify

multifd_save_cleanup() takes an Error ** argument and returns an
error code even though it can't actually fail.  Its callers
dutifully check for failure.  Remove the useless argument and return
value, and simplify the callers.

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Fei Li <fli@suse.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20190113140849.38339-4-lifei1214@126.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>