x86/hvm: do not register hpet mmio during s3 cycle

x86/hvm: do not register hpet mmio during s3 cycle

Do it once at domain creation (hpet_init).

Sleep -> Resume cycles will end up crashing an HVM guest with hpet as
the sequence during resume takes the path:
-> hvm_s3_suspend
  -> hpet_reset
    -> hpet_deinit
    -> hpet_init
      -> register_mmio_handler
        -> hvm_next_io_handler

register_mmio_handler will use a new io handler each time, until
eventually it reaches NR_IO_HANDLERS, then hvm_next_io_handler calls
domain_crash.

Signed-off-by: Eric Chanudet <chanudete@ainfosec.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>