Add vendor_device parameter for HVM guests

Add vendor_device parameter for HVM guests

The parameter determines which, if any, xen-pvdevice is specified on the
QEMU command line. The default value is 'none' which means no argument will
be passed. A value of 'xenserver' specifies a xen-pvdevice with device-id
0xc000 (the initial value in the xenserver namespace - see
docs/misc/pci-device-reservations.txt).

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- s/BUILD_INFO/BUILDINFO for consistency in LIBXL_HAVE define ]