qemu_firmware: Try to autofill for old style UEFI specification

While we discourage people to use the old style of specifying
UEFI for their domains (the old style is putting path to the FW
image under /domain/os/loader/ whilst the new one is using
/domain/os/@firmware), some applications might have not adapted
yet. They still rely on libvirt autofilling NVRAM path and
figuring out NVRAM template when using the old way (notably
virt-install does this). We must preserve backcompat for this
previously supported config approach. However, since we really
want distro maintainers to leave --with-loader-nvram configure
option and rely on JSON descriptors, we need to implement
autofilling of NVRAM template for the old way too.

Fedora: https://bugzilla.redhat.com/show_bug.cgi?id=1782778
RHEL: https://bugzilla.redhat.com/show_bug.cgi?id=1776949

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