libxl: Add function to determine device model type

libxl: Add function to determine device model type

This patch introduces a function to detect whether the specified
emulator is QEMU_XEN or QEMU_XEN_TRADITIONAL.  Detection is based on the
string "Options specific to the Xen version:" in '$qemu -help' output.
AFAIK, the only qemu containing that string in help output is the
old Xen fork (aka qemu-dm).

Note:
QEMU_XEN means a qemu that contains support for Xen.

QEMU_XEN_TRADITIONAL means Xen's old forked qemu 0.10.2

Signed-off-by: Jim Fehlig <jfehlig@suse.com>