x86/irq: KVM: Add helper for harvesting PIR to deduplicate KVM and posted MSIs

Now that posted MSI and KVM harvesting of PIR is identical, extract the
code (and posted MSI's wonderful comment) to a common helper.

No functional change intended.

Link: https://lore.kernel.org/r/20250401163447.846608-9-seanjc@google.com


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