Remove direct cpumask_t members from struct vcpu and struct domain

Remove direct cpumask_t members from struct vcpu and struct domain

The CPU masks embedded in these structures prevent NR_CPUS-independent
sizing of these structures.

Basic concept (in xen/include/cpumask.h) taken from recent Linux.

For scalability purposes, many other uses of cpumask_t should be
replaced by cpumask_var_t, particularly local variables of functions.
This implies that no functions should have by-value cpumask_t
parameters, and that the whole old cpumask interface (cpus_...())
should go away in favor of the new (cpumask_...()) one.

Signed-off-by: Jan Beulich <jbeulich@novell.com>