vdpa/pds: fixes for VF vdpa flr-aer handling

This addresses a couple of things found while testing the FLR and AER
handling with the VFs.
 - release irqs before calling vp_modern_remove()
 - make sure we have a valid struct pointer before using it to release irqs
 - make sure the FW is alive before trying to add a new device

Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Message-Id: <20240220011050.30913-1-shannon.nelson@amd.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>