libxc: allow creating domains without emulated devices

libxc: allow creating domains without emulated devices

Introduce a new flag in xc_dom_image that turns on and off the emulated
devices. This prevents creating the VGA hole, the hvm_info page and the
ioreq server pages. libxl unconditionally sets it to true for all HVM
domains at the moment.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>