virtio: clarify feature bit handling

virtio: clarify feature bit handling

Change the features from bit mask to bit number. This allows the
DPDK driver to use the definitions from Linux (yes the header
files already use a license compatiable with DPDK). This makes DPDK
driver handle future feature bit changes.

Get rid of double negative code in the feature bit intialization.
Instead just have a new define with the list of feature bits implemented.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Changchun Ouyang <changchun.ouyang@intel.com>