qemu: Warn when using vhost-user without shared memory
    
    When user configures vhost-user interface and forgets to also configure
    any shared memory, the search for the root cause of non-operational
    interface might take unpleasantly long time.  Let's enhance user
    experience by emitting a warning in the logs.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1266982
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>