vdpa: move backend_cap to vhost_vdpa_shared

vdpa: move backend_cap to vhost_vdpa_shared

Next patches will register the vhost_vdpa memory listener while the VM
is migrating at the destination, so we can map the memory to the device
before stopping the VM at the source.  The main goal is to reduce the
downtime.

However, the destination QEMU is unaware of which vhost_vdpa device will
register its memory_listener.  If the source guest has CVQ enabled, it
will be the CVQ device.  Otherwise, it  will be the first one.

Move the backend_cap member to VhostVDPAShared so all vhost_vdpa can use
it, rather than always in the first / last vhost_vdpa.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20231221174322.3130442-9-eperezma@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>