Update virtio-loopback-adapter, add multi-device support

Update virtio-loopback-adapter, add multi-device support

Updates [v2]:
Solve compatibility issue with virtio-loopback driver.
Update IOCTLs to be compatible with the latest driver version.

Updates [v1]:
This version adds multi-device support on virtio-loopback
adapter. With that the adapter is able to handle concurrently
requests coming from multiple vhost-user backends.

Bug-AGL: SPEC-4966

Change-Id: I42ed32b41daec3bd9c36176a49623a55174630e2
Signed-off-by: Timos Ampelikiotis <t.ampelikiotis@virtualopensystems.com>