Rename the DOMAIN_DEVICE_ITERATE_GRAPHICS flag.
It was introduced by commit dd45c271
with the intention to run the Validate callback even on the graphics
device.

However, enumerating every single device in virDomainDeviceIterateFlags
is unsustainable and what really was special about the graphics device
was the lack of DeviceInfo.

Rename the flag and iterate over more info-less devices. (and leases)
Signed-off-by: NJán Tomko <jtomko@redhat.com>