cpus: prepare new CpusAccel cpu accelerator interface

The new interface starts unused, will start being used by the
next patches.

It provides methods for each accelerator to start a vcpu, kick a vcpu,
synchronize state, get cpu virtual clock and elapsed ticks.

In qemu_wait_io_event, make it clear that APC is used only for HAX
on Windows.

Signed-off-by: Claudio Fontana <cfontana@suse.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

stubs/cpus-get-virtual-clock.c

@@ -0,0 +1,8 @@
+#include "qemu/osdep.h"
+#include "sysemu/cpu-timers.h"
+#include "qemu/main-loop.h"
+
+int64_t cpus_get_virtual_clock(void)
+{
+    return cpu_get_clock();
+}