more alarm timer cleanup

The timer_alarm_pending variable is related to the alarm timer but not
placed in the struct.  Also, in qemu_mod_timer the wrong flag was being
tested: the timer is rearmed in the alarm timer "bottom half", so the
right flag to test there is the "pending" flag.

Finally, I hoisted the NULL checks from alarm_has_dynticks to
host_alarm_handler.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
