KVM: x86: Add a helper to deduplicate I/O APIC EOI interception logic

Extract the vCPU specific EOI interception logic for I/O APIC emulation
into a common helper for userspace and in-kernel emulation in anticipation
of optimizing the "pending EOI" case.

No functional change intended.

Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20250304013335.4155703-3-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>