spapr_rtas: Prevent QEMU crash during hotplug without a prior device_add

If drmgr is used in the guest to hotplug a device before a device_add
has been issued via the QEMU monitor, QEMU segfaults in configure_connector
call. This occurs due to accessing of NULL FDT which otherwise would have
been created and associated with the DRC during device_add command.

Check for NULL FDT and return failure from configure_connector call.
As per PAPR+, an error value of -9003 seems appropriate for this failure.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
