libxl: make libxl_wait_for_device_model clearer

libxl: make libxl_wait_for_device_model clearer

at the moment libxl_wait_for_device_model waits on a xenstore watch
before checking the current value of the xenstore node, that might
contain already the value the function was looking for.

This patch changes libxl_wait_for_device_model so that it checks the
value of the xenstore node first, then waits for the watch.

xenstore watch automatically fire one time when you install them for
this exact same purpose, so the previous code is not wrong, but this
version is clearer.

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