net/virtio: keep Rx handler whatever the Tx queue config

net/virtio: keep Rx handler whatever the Tx queue config

Split use_simple_rxtx into use_simple_rx and use_simple_tx,
and ensure that only use_simple_tx is updated when txq flags
forces to use the standard Tx handler.

This change is also useful for next commit (disable simple Rx
path when Rx checksum is requested).

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Yuanhan Liu <yliu@fridaylinux.org>