pc: don't access fw cfg if NULL

commit f8c457b88d72a48989f190bc3d7b79f4f3b7d11c
     "pc: pass PCI hole ranges to Guests"
broke Xen as it has no fw_cfg.
Check for this configuration and boil out.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
