pds_vdpa: alloc irq vectors on DRIVER_OK

We were allocating irq vectors at the time the aux dev was probed,
but that is before the PCI VF is assigned to a separate iommu domain
by vhost_vdpa.  Because vhost_vdpa later changes the iommu domain the
interrupts do not work.

Instead, we can allocate the irq vectors later when we see DRIVER_OK and
know that the reassignment of the PCI VF to an iommu domain has already
happened.

Fixes: 151cc834 ("pds_vdpa: add support for vdpa and vdpamgmt interfaces")
Signed-off-by: Allen Hubbe <allen.hubbe@amd.com>
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230711042437.69381-5-shannon.nelson@amd.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>