hpet: fix and cleanup persistence of interrupt status

hpet: fix and cleanup persistence of interrupt status

There are several bugs in the handling of the ISR register:

- switching level->edge was not lowering the interrupt and
  clearing ISR

- switching on the enable bit was not raising a level-triggered
  interrupt if the timer had fired

- the timer must be kept running even if not enabled, in
  order to set the ISR flag, so writes to HPET_TN_CFG must
  not call hpet_del_timer()

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>