vmt(4) - implement vmbackup.* RPC call

[Takehiko NOZAKI]

O*BSD implement vmbackup RPC call by vfs_stall()

+        { "vmbackup.abort",        vmt_tclo_abortbackup },
+        { "vmbackup.snapshotDone", vmt_tclo_backupdone },
+        { "vmbackup.start 1",      vmt_tclo_startbackup },

‌

https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/dev/pv/vmt.c#rev1.14

N6 has vfs_{suspend,resume} for similer purpose.