KVM: use modern cpumask primitives, no cpumask_t on stack

We're getting rid on on-stack cpumasks for large NR_CPUS.

1) Use cpumask_var_t/alloc_cpumask_var.
2) smp_call_function_mask -> smp_call_function_many
3) cpus_clear, cpus_empty, cpu_set -> cpumask_clear, cpumask_empty,
   cpumask_set_cpu.

This actually generates slightly smaller code than the old one with
CONFIG_CPUMASKS_OFFSTACK=n.  (gcc knows that cpus cannot be NULL in
that case, where cpumask_var_t is cpumask_t[1]).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Avi Kivity <avi@redhat.com>