qemu: Allow UEFI paths to be specified at compile time

qemu: Allow UEFI paths to be specified at compile time

Up until now there are just two ways how to specify UEFI paths to
libvirt. The first one is editing qemu.conf, the other is editing
qemu_conf.c and recompile which is not that fancy. So, new
configure option is introduced: --with-loader-nvram which takes a
list of pairs of UEFI firmware and NVRAM store. This way, the
compiled in defaults can be passed during compile time without
need to change the code itself.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>