Commit 7456c4f5 introduced a regression by not reloading the backing
chain of a disk after snapshot. The regression was caused as
src->relPath was not set and thus the block commit code could not
determine the relative path.

This patch adds code that will load the backing store string if
VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT and store it in the correct place
when a snapshot is successfully completed.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1461303