spec: break out qemu-virtiofsd subpackage
    
    Pulled in by qemu-* but not qemu-*-core, like we do for device modules.
    
    There's a virtual Provides: vhostuser-backend(fs) indicating this
    packages is a vhost-user.json fs provider.
    
    Use that for the qemu dep, as in the future there will be alternate
    virtiofsd impl packages in Fedora
    
    Signed-off-by: Cole Robinson <crobinso@redhat.com>