libxl: Add a version check of QEMU for QMP commands

libxl: Add a version check of QEMU for QMP commands

On connection to QEMU via QMP, the version of QEMU is provided, store it
for later use.

Add a function qmp_qemu_check_version that can be used to check if QEMU
is new enough for certain fonctionnality. This will be used in a moment.

As it's a static function, it is commented out until first use, which is
in the next patch.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>