rust/hpet: change type of num_timers to usize

rust/hpet: change type of num_timers to usize

Remove the need to convert after every read of the BqlCell.  Because the
vmstate uses a u8 as the size of the VARRAY, this requires switching
the VARRAY to use num_timers_save; which in turn requires ensuring that
the num_timers_save is always there.  For simplicity do this by
removing support for version 1, which QEMU has not been producing for
~15 years.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>