vmm: validate icr value

Not all combinations of icr values are allowed. Neither Intel nor AMD
document what happens when an invalid value is written to the icr.
Ignore the IPI. So, the guest will note that the IPI wasn't delivered.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D36946
Sponsored by: Beckhoff Automation GmbH & Co. KG