introduce a cpumask with all bits set

introduce a cpumask with all bits set

There are several places in Xen allocating a cpumask on the stack and
setting all bits in it just to use it as an initial mask for allowing
all cpus.

Save the stack space and omit the need for runtime initialization by
defining a globally accessible cpumask_all variable.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>