qemu: Assume QEMU_CAPS_VIRTIO_NET_HOST_MTU

qemu: Assume QEMU_CAPS_VIRTIO_NET_HOST_MTU

Introduced in QEMU's commit of v2.9.0-rc0~162^2~10 the .host_mtu
attribute of virtio-net device is always available for all QEMU
versions we support (4.2.0, currently). Therefore, we can assume
the capability is always set and thus doesn't need to be checked
for.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>