Add a helper to retrieve the highest pending vector given a Posted
Interrupt descriptor.  While the actual operation is straightforward, it's
surprisingly easy to mess up, e.g. if one tries to reuse lapic.c's
find_highest_vector(), which doesn't work with PID.PIR due to the APIC's
IRR and ISR component registers being physically discontiguous (they're
4-byte registers aligned at 16-byte intervals).

To make PIR handling more consistent with respect to IRR and ISR handling,
return -1 to indicate "no interrupt pending".

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240607172609.3205077-2-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>