qemu: Introduce basic skeleton for parsing firmware description

qemu: Introduce basic skeleton for parsing firmware description

The firmware description is a JSON file which follows
specification from qemu.git/docs/interop/firmware.json. The
description file basically says: Firmware file X is {bios|uefi},
supports these targets and machine types, requires these features
to be enabled on qemu cmd line and this is how you put it onto
qemu cmd line.

The firmware.json specification covers more (i.e. how to select
the right firmware) but that will be covered and implemented in
next commits.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>