vdpa/mlx5: support direct HW notifications

vdpa/mlx5: support direct HW notifications

Add support for the next 2 callbacks:
get_vfio_device_fd and get_notify_area.

This will allow direct HW doorbell ringing from guest and will save CPU
usage in host.

By this patch, the QEMU will map the physical address of the virtio
device in guest directly to the physical address of the HW device
doorbell.

The guest doorbell write is 2 bytes transaction while some Mellanox nics
support only 4 bytes transactions.

Remove ConnectX-5 and BF1 devices support which don't support 2B
doorbell writes for HW triggering.

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