conf: Add rom.enabled attribute for PCI devices
    
    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: Andrea Bolognani <abologna@redhat.com>