Simplify reporting the error if backing chain is broken for further
callers by extracting it into a separate function.

When the user provides backing chain, we don't need the full support for
traversing the backing chain. This patch adds a feature check for the
virStorageSourceAccess API.

The 'file access' module of the storage driver has few feature checks to
determine whether libvirt supports given storage driver method. The code
to retrieve the driver struct needed for the check is the same so it can
be extracted.

Express a properly terminated backing chain by putting a
virStorageSource of type VIR_STORAGE_TYPE_NONE in the chain. The newly
used helpers simplify this greatly.

The change fixes a bug as formatting an incomplete backing chain and
parsing it back would end up in expressing a terminated chain since
src->backingStoreRaw was not populated. By relying on the terminator
object this can be now processed appropriately.

Add helpers that will simplify checking if a backing file is valid or
whether it has backing store. The helper virStorageSourceIsBacking
returns true if the given virStorageSource is a valid backing store
member. virStorageSourceHasBacking returns true if the virStorageSource
has a backing store child.

Adding these functions creates a central points for further refactors.

The backing store indexes were not bound to the storage sources in any
way. To allow us to bind a given alias to a given storage source we need
to save the index in virStorageSource. The backing store ids are now
generated when detecting the backing chain.

Since we don't re-detect the backing chain after snapshots, the
numbering needs to be fixed there.

Our backing probing code handles directory file types properly in
virStorageFileGetMetadataRecurse(), by that I mean it leaves them
alone.  However its caller, the virStorageFileGetMetadata() resets the
type to raw before probing, without even checking the type.  We need
to special-case TYPE_DIR in order to achieve desired results.

Also, in order to properly test this, we need to stop resetting format
of volumes in tests for TYPE_DIR (probably the reason why we didn't
catch that and why the test data didn't need to be modified).

Partially-resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1443434Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

Allow specifying offset to read an arbitrary position in the file. This
warrants a rename to virStorageFileRead.

The helper methods for actually accessing the storage objects don't
really belong to the main storage driver implementation file. Split them
out.