qemu: Automatically create NVRAM store

qemu: Automatically create NVRAM store

When using split UEFI image, it may come handy if libvirt manages per
domain _VARS file automatically. While the _CODE file is RO and can be
shared among multiple domains, you certainly don't want to do that on
the _VARS file. This latter one needs to be per domain. So at the
domain startup process, if it's determined that domain needs _VARS
file it's copied from this master _VARS file. The location of the
master file is configurable in qemu.conf.

Temporary, on per domain basis the location of master NVRAM file can
be overridden by this @template attribute I'm inventing to the
<nvram/> element. All it does is holding path to the master NVRAM file
from which local copy is created. If that's the case, the map in
qemu.conf is not consulted.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>