stubdom: fixes to compile with qemu-xen

stubdom: fixes to compile with qemu-xen

This adds fixes to the stub domain build into compiling Ian Jackson's
qemu-xen.  The most notable change is that mini-os headers now
#include each other through a mini-os/ prefix, so that we can turn all
-I into -isystem and still be sure that we include Mini-OS headers
(and not qemu's console.h or blktaplib's list.h for instance...).

Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>