Use virFileOperation hook function in virStorageBackendFileSystemVolBuild

Use virFileOperation hook function in virStorageBackendFileSystemVolBuild

There were a few operations on the storage volume file that were still
being done as root, which will fail if the file is on a root-squashed
NFS share. The result was that attempts to create a storage volume of
type "raw" on a root-squashed NFS share would fail.

This patch uses the newly introduced "hook" function in
virFileOperation to execute all those file operations in the child
process that's run under the uid that owns the file (and, presumably,
has permission to write to the NFS share)

* src/storage/storage_backend.c: use virFileOperation() in
  virStorageBackendCreateRaw, turning virStorageBackendCreateRaw()
  into a new createRawFileOpHook() hook