libxl: allow the creation of HVM domains without a device model.

libxl: allow the creation of HVM domains without a device model.

Replace the firmware loaded into HVM guests with an OS kernel. Since the HVM
builder now uses the PV xc_dom_* set of functions this kernel will be parsed
and loaded inside the guest like on PV, but the container is a pure HVM
guest.

Also, if device_model_version is set to none or a device model for the
specified domain is not present unconditinally set the nic type to
LIBXL_NIC_TYPE_VIF.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>