bhyve: save/restore pir_desc

Failing to preserve pir_desc can result in pending interrupts being lost
on resume leading to a hung VM.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: vStack
Differential Revision: https://reviews.freebsd.org/D35447

(cherry picked from commit c543e09f1f8ef98f201f7dd3f34ae023d61dfa83)