libxl: adding support to use -machine option of QEMU.

This can be use later to switch to a Q35 based emulated machine. It can
also be used on a recent QEMU to avoid adding the pci-platform-device
used to setup PV drivers in the guest.
libxl: Handle xen_platform_pci=0 case with qemu-xen.

TOWRITE with:
handle some case were qemu_machine is set in the config file.