Add more elements for tuning the virtiofsd daemon
and the vhost-user-fs device:

  <driver type='virtiofs' queue='1024' xattr='on'>
    <binary path='/usr/libexec/virtiofsd'>
      <cache mode='always'/>
      <lock posix='off' flock='off'/>
    </binary>
  </driver>
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NMasayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Reviewed-by: NPeter Krempa <pkrempa@redhat.com>
Tested-by: NAndrea Bolognani <abologna@redhat.com>