conf: let snapshots share disk source struct

conf: let snapshots share disk source struct

Now that we have a common struct, it's time to start using it!
Since external snapshots make a longer backing chain, it is
only natural to use the same struct for the file created by
the snapshot as what we use for <domain> disks.

* src/conf/snapshot_conf.h (_virDomainSnapshotDiskDef): Use common
struct instead of open-coded duplicate fields.
* src/conf/snapshot_conf.c (virDomainSnapshotDiskDefClear)
(virDomainSnapshotDiskDefParseXML, virDomainSnapshotAlignDisks)
(virDomainSnapshotDiskDefFormat)
(virDomainSnapshotDiskGetActualType): Adjust clients.
* src/qemu/qemu_conf.c (qemuTranslateSnapshotDiskSourcePool):
Likewise.
* src/qemu/qemu_driver.c (qemuDomainSnapshotDiskGetSourceString)
(qemuDomainSnapshotCreateInactiveExternal)
(qemuDomainSnapshotPrepareDiskExternalOverlayActive)
(qemuDomainSnapshotPrepareDiskExternal)
(qemuDomainSnapshotPrepare)
(qemuDomainSnapshotCreateSingleDiskActive): Likewise.
* src/storage/storage_driver.c
(virStorageFileInitFromSnapshotDef): Likewise.

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