conf: require target for external virtiofsd

When adding support for externally launched virtiofsd,
I was too liberal and did not require a target.

But the target is required, because it's passed to the
QEMU device, not to virtiofsd.

https://bugzilla.redhat.com/show_bug.cgi?id=1969232

Fixes: 12967c3e13
Fixes: 56dcdec1ac
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

tests/qemuxml2argvdata/vhost-user-fs-sock.xml

@@ -29,6 +29,7 @@
     <filesystem type='mount'>
       <driver type='virtiofs' queue='1024'/>
       <source socket='/tmp/sock'/>
+      <target dir='tag'/>
       <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
     </filesystem>
     <input type='mouse' bus='ps2'/>