hvm/hpet: correctly gate the virtual HPET on HVM_PARAM_HPET_ENABLE

hvm/hpet: correctly gate the virtual HPET on HVM_PARAM_HPET_ENABLE

c/s 3f8e22de7 "x86 hvm: Allow HPET to be configured as a per-domain config
option" introduced the parameter to conditionally enable the HPET.

However, having the check in hpet_range() does not have the intended effect.
As currently implemented, when the HPET is disabled, the range is not claimed
and an ioreq is forwarded to qemu, which implements an HPET itself.

Properly disable the HPET by always claiming the range, dropping writes and
reading ~0.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>