net/virtio: define avail and used flags as constants

net/virtio: define avail and used flags as constants

We are consistently passing 1 as the argument in the data path,
so there is no need to define avail/used flags as function-like
macros anymore. This patch changes the avail and used flags to
constants. And a frequently used combination is also introduced.

Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Jens Freimann <jfreimann@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>