vmm: validate icr value

[corvink]

Summary

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.

[jhb]

sys/amd64/vmm/io/vlapic.c

992

We may want to explicitly comment here that level-deassert INIT is being purposefully ignored because it is a defined thing (so "valid" in one sense), but also something we should ignore.

[corvink]

• check for level deassert INIT and comment why we handle it as invalid