p9fs.4: MLINK to virtio_p9fs.4

Technically, virtio_p9fs is an emulated device that masquerades
as a p9fs mount, but it does not make sense to have two separate manual
pages.

Reviewed by: bnovkov, dfr
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D57013

(cherry picked from commit 30f500e97b57d36ebb8483cb8b8921507350cd0f)