When creating a snapshot with _REUSE_EXTERNAL when the pre-created image
does not directly link to the current active layer libvirt would
re-detect the backing chain incorrectly and it would not match with
qemu's view. Since the configuration is an operator mistake, document
that only the top layer image gets inserted.

The error output of snapshot-revert should be more friendly.
There is no need to show virDomainRevertToSnapshot to user.
virReportError already includes __FUNCTION__ information in a
separate member of the struct, so repeating it in the message is
redundant and leads to situations where higher level code ends up
reporting the lower level name. We correctly converted the error
output making it more succinct and user-friendly.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1086726

Introduce a src/libvirt-domain-snapshot.c file to hold all the
methods related to the virDomainSnapshot type.