Deciding if a user string represents a local file instead of a
network path is an operation worth exposing directly, particularly
since the next patch will be removing a redundant variable that
was caching the information.

* src/util/virstoragefile.h (virStorageIsFile): New declaration.
* src/util/virstoragefile.c (virBackingStoreIsFile): Rename...
(virStorageIsFile): ...export, and allow NULL input.
(virStorageFileGetMetadataInternal)
(virStorageFileGetMetadataRecurse, virStorageFileGetMetadata):
Update callers.
* src/conf/domain_conf.c (virDomainDiskDefForeachPath): Use it.
* src/storage/storage_backend_fs.c (virStorageBackendProbeTarget):
Likewise.
* src/libvirt_private.syms (virstoragefile.h): Export function.
Signed-off-by: NEric Blake <eblake@redhat.com>