NFS: Use network-online.target instead of remote-fs-pre.target

Turns out that remote-fs-pre.target is not actually "wanted" anywhere,
so statd is not started before remote filesystems are mounted.  But
remote filesystems do "want" network-online.target, so we can use that
to pull in statd and idmapd.

Not sure if this is really the right thing to do, but it works for
now.  Background:

  https://bugzilla.redhat.com/show_bug.cgi?id=787314

http://hydra.nixos.org/build/5542230
Start sm-notify/statd atfer nfsd

The README of nfs-utils explains that we must not notify clients
before nfsd is running, otherwise they may fail to reclaim their
locks.  OTOH it's allowed but not required to start "rpc.statd
--no-notify" before nfsd.  So for simplicity we do both after starting
nfsd.
Don't pull in the old qemu-kvm
NFS test: Set a fixed fsid

Otherwise clients get a "Stale NFS handle" error after the server
reboots.  Not clear why this is the case, since the fsid shouldn't
change across reboots...
Add a test for NFSv4

However it currently fails because locking seems to be broken with
NFSv4.