The current use of virStorageFileMetadata is awkward; to learn
some of the information about a child node, you have to read
fields in the parent node.  This does not lend itself well to
modifying backing chains (whether inserting a new node in the
chain, or consolidating existing nodes); better would be to
learn about a child node directly in that node.  This patch
sets up some new fields which contain redundant information,
although not necessarily in the final desired state for the
new fields (see the next patch for actual tests of what is there
now).  Then later patches will do any refactoring necessary to
get the fields to their desired states, and update clients to
get the information from the new fields, so we can finally
delete the fields that are tracking information about the wrong
node.

More concretely, compare these three example backing chains:

good <- one
missing <- two
gluster://server/vol/img <- three

Pre-patch, querying the chains gives:
{ .backingStore = "/path/to/good",
  .backingStoreRaw = "good",
  .backingStoreIsFile = true,
  .backingStoreFormat = VIR_STORAGE_FILE_RAW,
  .backingMeta = {
    .backingStore = NULL,
    .backingStoreRaw = NULL,
    .backingStoreIsFile = false,
    .backingMeta = NULL,
  }
}
{ .backingStore = NULL,
  .backingStoreRaw = "missing",
  .backingStoreIsFile = false,
  .backingStoreFormat = VIR_STORAGE_FILE_NONE,
  .backingMeta = NULL,
}
{ .backingStore = "gluster://server/vol/img",
  .backingStoreRaw = NULL,
  .backingStoreIsFile = false,
  .backingStoreFormat = VIR_STORAGE_FILE_RAW,
  .backingMeta = NULL,
}

Deciding whether to ignore a missing backing file (as in virsh
vol-dumpxml) or report an error (as in security manager sVirt
labeling) requires reading multiple fields.  Plus, the format
is hard-coded to treat all network protocols as end-of-the-chain,
as if they were raw.  By the end of this patch series, the goal
is to instead represent these three situations as:

{ .path = "one",
  .canonPath = "/path/to/one",
  .type = VIR_STORAGE_TYPE_FILE,
  .format = VIR_STORAGE_FILE_QCOW2,
  .backingStoreRaw = "good",
  .backingMeta = {
    .path = "good",
    .canonPath = "/path/to/good",
    .type = VIR_STORAGE_TYPE_FILE,
    .format = VIR_STORAGE_FILE_RAW,
    .backingStoreRaw = NULL,
    .backingMeta = NULL,
  }
}
{ .path = "two",
  .canonPath = "/path/to/two",
  .type = VIR_STORAGE_TYPE_FILE,
  .format = VIR_STORAGE_FILE_QCOW2,
  .backingStoreRaw = "missing",
  .backingMeta = NULL,
}
{ .path = "three",
  .canonPath = "/path/to/three",
  .type = VIR_STORAGE_TYPE_FILE,
  .format = VIR_STORAGE_FILE_QCOW2,
  .backingStoreRaw = "gluster://server/vol/img",
  .backingMeta = {
    .path = "gluster://server/vol/img",
    .canonPath = "gluster://server/vol/img",
    .type = VIR_STORAGE_TYPE_NETWORK,
    .format = VIR_STORAGE_FILE_RAW,
    .backingStoreRaw = NULL,
    .backingMeta = NULL,
  }
}

or, for the second file, maybe also allowing:
{ .path = "two",
  .canonPath = "/path/to/two",
  .type = VIR_STORAGE_TYPE_FILE,
  .format = VIR_STORAGE_FILE_QCOW2,
  .backingStoreRaw = "missing",
  .backingMeta = {
    .path = "missing",
    .canonPath = NULL,
    .type = VIR_STORAGE_TYPE_NONE,
    .format = VIR_STORAGE_FILE_NONE,
    .backingStoreRaw = NULL,
    .backingMeta = NULL,
  }
}

* src/util/virstoragefile.h (_virStorageFileMetadata): Add
path, canonPath, relDir, type, and format fields.  Reorder
existing fields, and add lots of comments.
* src/util/virstoragefile.c (virStorageFileFreeMetadata): Clean
new fields.
(virStorageFileGetMetadataInternal)
(virStorageFileGetMetadataFromFDInternal): Start populating new
fields.
Signed-off-by: NEric Blake <eblake@redhat.com>