kvm: avoid unused variable warning for UP builds

The uniprocessor version of smp_call_function_many does not evaluate
all of its argument, and the compiler emits a warning about "wait"
being unused.  This breaks the build on architectures for which
"-Werror" is enabled by default.

Work around it by moving the invocation of smp_call_function_many to
its own inline function.

Reported-by: Paul Mackerras <paulus@ozlabs.org>
Cc: stable@vger.kernel.org
Fixes: 7a97cec2
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>