KVM x86 APIC changes for 6.7:

 - Purge VMX's posted interrupt descriptor *before* loading APIC state when
   handling KVM_SET_LAPIC.  Purging the PID after loading APIC state results in
   lost APIC timer IRQs as the APIC timer can be armed as part of loading APIC
   state, i.e. can immediately pend an IRQ if the expiry is in the past.

 - Clear the ICR.BUSY bit when handling trap-like x2APIC writes.  This avoids a
   WARN, due to KVM expecting the BUSY bit to be cleared when sending IPIs.