migration: unify error handling for process_incoming_migration_co

migration: unify error handling for process_incoming_migration_co

In the current code, if process_incoming_migration_co() fails we do
the same error handing: set the error state, close the source file,
do the cleanup for multifd, and then exit(EXIT_FAILURE). To make the
code clearer, add a "goto fail" to unify the error handling.

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Fei Li <fli@suse.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190113140849.38339-6-lifei1214@126.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>