timer: make qemu_clock_enable sync between disable and timer's cb

After disabling the QemuClock, we should make sure that no QemuTimers
are still in flight. To implement that with light overhead, we resort
to QemuEvent. The caller of disabling will wait on QemuEvent of each
timerlist.

Note, qemu_clock_enable(foo,false) can _not_ be called from timer's cb.
Also, the callers of qemu_clock_enable() should be protected by the BQL.

Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
