libxl: Allow running qemu-xen in stubdomain

libxl: Allow running qemu-xen in stubdomain

Do not prohibit anymore using stubdomain with qemu-xen.
To help distingushing MiniOS and Linux stubdomain, add helper inline
functions libxl__stubdomain_is_linux() and
libxl__stubdomain_is_linux_running(). Those should be used where really
the difference is about MiniOS/Linux, not qemu-xen/qemu-xen-traditional.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>