pci: don't ignore invalid parameter for pci_register_bar().

Abort when invalid value for region_num is passed to pci_register_bar.
That is caller's bug. Abort instead of silently ignoring invalid value.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
