The attribute can be used to disable ROM loading completely
for a device.

This might be needed because, even when the guest is configured
such that the PCI ROM will not be loaded in the PCI BAR, some
hypervisors (eg. QEMU) might still make it available to the
guest in a form (eg. fw_cfg) that some firmwares (eg. SeaBIOS)
will consume, thus not achieving the desired result.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>