net/virtio: move backend type selection to ethdev

net/virtio: move backend type selection to ethdev

This is a preparation patch with no functional change.

Use an enum instead of a boolean for the backend type.
Move the detection logic to the ethdev layer (where it is needed for the
first time).
The virtio_user_dev stores the backend type in the virtio_user_dev
struct so the type is only determined once

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>