watchdog: fix deadlock with -watchdog-action pause

qemu_clock_enable says:

/* Disabling the clock will wait for related timerlists to stop
 * executing qemu_run_timers.  Thus, this functions should not
 * be used from the callback of a timer that is based on @clock.
 * Doing so would cause a deadlock.
 */

and it indeed does: vm_stop uses qemu_clock_enable on QEMU_CLOCK_VIRTUAL
and watchdogs are based on QEMU_CLOCK_VIRTUAL, and we get a deadlock.

Use qemu_system_vmstop_request_prepare()/qemu_system_vmstop_request()
instead; yet another alternative could be a BH.

I checked other occurrences of vm_stop and they should not have this
problem.  RUN_STATE_IO_ERROR could in principle (it depends on the
code in the drivers) but it has been fixed by commit 2bd3bce8, "block:
asynchronously stop the VM on I/O errors", 2014-06-05.

Tested-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>