The pba_page_offset field was removed. It was only used by the
passthrough code, and I believe the snapshot code currently does not
handle passthrough devices, so the field perhaps shouldn't have been
there to begin with.

I believe this change breaks compatibility with snapshots created by a
bhyve that doesn't have this change. I'm not sure what guarantees we
provide there, but if necessary we could write a dummy value instead.

Fixes: 7fa233534736 ("bhyve: Map the MSI-X table unconditionally for passthrough")