qemu: hotplug: Setup host side of VDPA device for disk hotplug

qemu: hotplug: Setup host side of VDPA device for disk hotplug

The code which opens the VDPA device and prepares it for FD passing was
not called in the hotplug code path, preventing hotplug of VDPA disks
with:

 error: internal error: argument key 'path' must not have null value

Use the new helper qemuProcessPrepareHostStorageDisk to setup the VDPA
definition.

Closes: https://gitlab.com/libvirt/libvirt/-/issues/539
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>