qemu: Don't drop firmware type/features information

qemu: Don't drop firmware type/features information

Now that we no longer reject configurations that include both
this information and explicit firmware details, as long of
course as everything is internally consistent, and that we've
ensured that we produce maximally compatible XML on migration,
we can stop stripping this information at the end of the
firmware selection process.

There are several advantages to keeping this information around:

  * if the user wants to change the firmware configuration for
    an existing VM, they can simply drop the <loader> and
    <nvram> elements, tweak the firmware autoselection parameters
    and let libvirt pick a firmware that matches on the new
    requirements;

  * management applications can inspect the XML and easily
    figure out firmware-related information without having to
    reverse-engineer them based on some opaque paths.

Overall, this change makes things more transparent and easier to
understand. The improvement is so significant that, in a
follow-up commit, we're going to ensure that this information is
available in even more cases.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>