qemu: command: Don't attempt to set backend MTU for networks which don't use host...

qemu: command: Don't attempt to set backend MTU for networks which don't use host backend directly

Attempting to set MTU for network types which don't actually use the
network device on the host results in a failure. The 'mtu' property is
also used e.g. for the 'host_mtu' property of e.g. 'virtio-net-pci'
which is applied even in vhost-user mode.

Use the existing switch which selects devices without a network device
backend on the host side and skip setting the MTU.

Tested by running 'passt' in vhost-user mode manually:

 passt -f --vhost-user -s /tmp/vh.sock

and the following XML:

 <interface type="vhostuser">
   <mac address="52:54:00:3d:91:97"/>
   <source type="unix" path="/tmp/vh.sock" mode="client"/>
   <model type="virtio"/>
   <mtu size="9999"/>
   <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x0"/>
 </interface>

The OS in the guest reports MTU 9999.

Closes: https://gitlab.com/libvirt/libvirt/-/issues/717
Closes: https://gitlab.com/libvirt/libvirt/-/issues/192
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>