vdpa/mlx5: map doorbell

vdpa/mlx5: map doorbell

The HW supports only 4 bytes doorbell writing detection.
The virtio device set only 2 bytes when it rings the doorbell.

Map the virtio doorbell detected by the virtio queue kickfd to the HW
VAR space when it expects to get the virtio emulation doorbell.

Use the EAL interrupt mechanism to get notification when a new event
appears in kickfd by the guest and write 4 bytes to the HW doorbell space
in the notification callback.

Signed-off-by: Matan Azrad <matan@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>