configure: Fix copy-paste error in vstorage backend check

The 'fs' backend m4 code was copied, but one of the condition were not
changed which resulted into:

configure:    Gluster: yes
configure:        ZFS: yes
configure: Virtuozzo storage: check

Make the code turn on the virtuozzo driver automatically if the tools
are found.

m4/virt-storage-vstorage.m4

@@ -49,7 +49,7 @@ AC_DEFUN([LIBVIRT_STORAGE_CHECK_VSTORAGE], [
         with_storage_vstorage=no
       fi
 
-      if test "$with_storage_fs" = "check" ; then
+      if test "$with_storage_vstorage" = "check" ; then
         with_storage_vstorage=yes
       fi
     fi