vhost: use linked list for vDPA devices

vhost: use linked list for vDPA devices

There is no more notion of device ID outside of vdpa.c.
We can now move from array to linked-list model for keeping
track of the vDPA devices.

There is no point in using array here, as all vDPA API are
used from the control path, so no performance concerns.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Adrián Moreno <amorenoz@redhat.com>