qemu: Assume QEMU_CAPS_VIRTIO_NET_TX_QUEUE_SIZE

qemu: Assume QEMU_CAPS_VIRTIO_NET_TX_QUEUE_SIZE

Introduced in QEMU's commit of v2.10.0-rc0~95^2~20 the
.tx_queue_size 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>