qemu: Add support for gic-version machine option

qemu: Add support for gic-version machine option

Support for GICv3 has been recently introduced in qemu using gic-version
option for the 'virt' machine. The option can actually take values of
'2', '3' and 'host', however, since in libvirt this is a numeric
parameter, we limit it only to 2 and 3. Value of 2 is not added to the
command line in order to keep backward compatibility with older qemu
versions.

Signed-off-by: Pavel Fedin <p.fedin@samsung.com>