libxl: enabling upstream qemu as pure pv backend.

libxl: enabling upstream qemu as pure pv backend.

This patch makes device_model_{version,override} work for pure pv
guest, so that users can specify upstream qemu as pure pv backend
other than traditional qemu-xen.

This patch also adds device_model_args_{pv,hvm} options for pv and
hvm guest respectively.

Internally, original libxl__create_xenpv_qemu allocates a new empty
dm_info (struct libxl_device_model_info) for every xenpv qemu created.
Now the caller of libxl__create_xenpv_qemu is responsible for
allocating this dm_info.

Signed-off-by: Wei Liu <liuw@liuw.name>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>