pci romfiles: add property, add default to PCIDeviceInfo

pci romfiles: add property, add default to PCIDeviceInfo

This patch adds a romfile property to the pci bus.  It allows to specify
a romfile to load into the rom bar of the pci device.  The default value
comes from a new field in PCIDeviceInfo.  The property allows to change
the file and also to disable the rom loading using an empty string.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit 8c52c8f320b27684ec3b1a649925b75af376b1f7)