qemu: Generate NVRAM path in more cases

qemu: Generate NVRAM path in more cases

Right now, we only generate it after finding a matching entry
either among firmware descriptors or in the legacy firmware
list.

Even if the domain is configured to use a custom firmware build
that we know nothing about, however, we should still automatically
generate the NVRAM path instead of requiring the user to provide
it manually.

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