conf: Add error checking to virDomainSnapshotDiskDefFormat

conf: Add error checking to virDomainSnapshotDiskDefFormat

Commit id '43f2ccdc' called virDomainDiskSourceDefFormatInternal
rather than formatting the the disk source inline. However, it
did not handle the case where the helper failed. Over time the
helper has been renamed to virDomainDiskSourceFormat. Similar to
other consumers, if virDomainDiskSourceFormat fails, then the
formatting could be off, so it's better to fail than to continue
on with some possibly bad data. Alter the function and the caller
to check status and jump to error in that case.

Found by Coverity

Signed-off-by: John Ferlan <jferlan@redhat.com>