x86/vhpet: check that the set interrupt route is valid

x86/vhpet: check that the set interrupt route is valid

The value written by the guest must be valid according to the mask
provided in the interrupt routing capabilities register. If the
interrupt is not valid set it to the first valid IRQ in the
capabilities field if the timer is enabled, else just clear the field.

Also refuse to start any timer that has an invalid interrupt route.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>