qemu: avoid calling the balloon info command if disabled

qemu: avoid calling the balloon info command if disabled

 Basically a followup of the previous patch about balloon desactivation
if desactivated, to not ask for balloon information to qemu as we will
just get an error back.
 This can make a huge difference in the time needed for domain
information or list when a machine is loaded, and balloon has been
desactivated in the guests.

* src/qemu/qemu_driver.c: do not get the balloon info if the balloon
  suppor is disabled