x86: Add struct ioapic_intsrc.io_valid

As of this commit, io_valid is always set to 1; but a future commit
will set it to 0, at which point IOART_INTMSET will be set to forcibly
disable interrupt sources regardless of whether they are requested to
be "masked".

Reviewed by: kib
MFC after: 3 weeks
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D56006