Detect NFS server by service name on Arch

Fixes #7629.
nfs-server.service seems to load kernel modules it needs itself, while nfsd appears in `/proc/filesystems` only after the kernel module has been loaded, so vagrant fails to detect NFS server until it's started first time after the system has booted. This checks if the NFS service actually exists and hopes that it'll figure the stuff out itself.

`list-unit-files` is utilized rather than `list-units` because systemd seems to not list units that are disabled: https://lists.fedoraproject.org/pipermail/devel/2011-November/159117.html

plugins/hosts/arch/cap/nfs.rb

@@ -11,7 +11,7 @@ module VagrantPlugins
         end
 
         def self.nfs_installed(environment)
-          Kernel.system("grep -Fq nfsd /proc/filesystems")
+          Kernel.system("systemctl --no-pager --no-legend --plain list-unit-files --all --type=service | grep --fixed-strings --quiet nfs-server.service")
         end
       end
     end