migration-test: Make wait_command() return the "return" member

All callers of wait_command() are only interested in the success
response's "return" member.  Lift its extraction into wait_command().

Cc: Juan Quintela <quintela@redhat.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-16-armbru@redhat.com>