driver config: Introduce virFirmware object

driver config: Introduce virFirmware object

The virQEMUDriverConfig object contains lists of
loader:nvram pairs to advertise firmwares supported by
by the driver, and qemu_conf.c contains code to populate
the lists, all of which is useful for other drivers too.

To avoid code duplication, introduce a virFirmware object
to encapsulate firmware details and switch the qemu driver
to use it.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>