snapshot: Factor out post-parse code

snapshot: Factor out post-parse code

Move the non-deterministic code that sets snapshot properties
independently of what the incoming XML described to instead live in a
default post-parse function common to virDomainMoment (as checkpoints
will also reuse it in later patches). This patch is just code motion,
with no difference to any callers; but the next patch will further
refactor things to allow for a per-driver override, used by the
testsuite to perform deterministic post-parse actions for better
coverage of parser/formatter code.

Note that the post-parse code is intentionally not run during a
snapshot redefine, since that code path already requires a valid
snapshot name and creation time from the XML.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>