libxenlight: implement stubdom support

libxenlight: implement stubdom support

this patch implements stubdom support for libxenlight:

- it adds two functions to find the stubdom domid of a domain and to
figure out if a certain domain is actually a stubdom;

- it moves all the device init functions from xl.c to libxl.c because
they are needed to setup the devices of stubdoms;

- it fixes some bugs in the pci setup that prevented pci passthrough
from working correctly with stubdoms.

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