libxl: Replace deprecated QMP command by "query-cpus-fast"

libxl: Replace deprecated QMP command by "query-cpus-fast"

We use the deprecated QMP command "query-cpus" which is removed in the
QEMU 6.0 release. There's a replacement which is "query-cpus-fast",
and have been available since QEMU 2.12 (April 2018).

This patch try the new command first and when the command isn't
available, it fall back to the deprecated one so libxl still works
with older QEMU versions.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>