Introduce a new 'virtiofs' driver type for filesystem.

<filesystem type='mount' accessmode='passthrough'>
  <driver type='virtiofs'/>
  <source dir='/path'/>
  <target dir='mount_tag'>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</filesystem>
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NPeter Krempa <pkrempa@redhat.com>
Tested-by: NAndrea Bolognani <abologna@redhat.com>