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>