Add unprivileged systemd service for virtiofsd

to share directory tree with unprivileged QEMU VMs. Configure virtio-fs
socket path /run/user/$UID/virtiofsd-$name.sock in libvirt DOMAIN XML.

This relies on RootlessKit used by Podman and Docker.

virtiofsd/.config/systemd/user/virtiofsd@.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=Virtio FS Daemon instance %i
+
+[Service]
+ExecStartPre=/usr/bin/mkdir -p %h/shared/%i
+ExecStart=/usr/bin/rootlesskit /usr/lib/virtiofsd --socket-path %t/virtiofsd-%i.sock --shared-dir %h/shared/%i
+Restart=always
+RestartSec=2s
+
+[Install]
+WantedBy=default.target