conf: manage disk source by struct instead of pieces

conf: manage disk source by struct instead of pieces

Now that we have a dedicated type for representing a disk source,
we might as well parse and format directly into that type instead
of piecemeal into pointers to members of the type.

* src/conf/domain_conf.h (virDomainDiskSourceDefFormatInternal)
(virDomainDiskSourceDefParse): Rename...
(virDomainDiskSourceFormat, virDomainDiskSourceParse): ...and
compress signatures.
* src/conf/domain_conf.c (virDomainDiskSourceParse)
(virDomainDiskSourceFormat): Rewrite to use common struct.
(virDomainDiskSourceDefFormat): Delete.
(virDomainDiskDefParseXML, virDomainDiskDefFormat): Update
callers.
* src/conf/snapshot_conf.c (virDomainSnapshotDiskDefParseXML)
(virDomainSnapshotDiskDefFormat): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>