vDPA/ifcvf: allocate the adapter in dev_add()

The adapter is the container of the vdpa_device,
this commits allocate the adapter in dev_add()
rather than in probe(). So that the vdpa_device()
could be re-created when the userspace creates
the vdpa device, and free-ed in dev_del()

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Cc: stable@vger.kernel.org
Message-Id: <20221125145724.1129962-11-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>