storage: Fix build due to recent storage backend code movement

storage: Fix build due to recent storage backend code movement

Commit id '5f07c3c07' broke the freebsd build in the libvirt CI test
environment because the UMOUNT was not defined unless WITH_STORAGE_FS
is defined.

So remove the virStorageBackendUmountLocal from storage_util.c,h and
restore the code back in the storage_backend_fs.c and _vstorage.c
modules.