hpet: recover timer offset correctly

HPET saves its state by calculating the current time and recovers timer
offset using this calculated value. But these calculations include
divisions and multiplications. Therefore the timer state cannot be recovered
precise enough.
This patch introduces saving of the original value of the offset to
preserve the determinism of the timer.

Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Signed-off-by: Maria Klimushenkova <maria.klimushenkova@ispras.ru>
Reviewed-by: Juan Quintela <quintela@redhat.com>

--
v3: Added compat property for correct migration.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>