libxenlight: wait for pv qemu initialization

libxenlight: wait for pv qemu initialization

this patch makes libxl_create_stubdom wait for pv qemu to be properly
initialized before unpausing the stubdom.
A new libxl_device_model_starting pointer is used to wait for pv qemu
initialization while the libxl_device_model_starting pointer given by
the user is initialized to a new structure with an empty for_spawn
member, because nothing that was spawn has to be waited for anymore.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>