snapshot: Allow for post-parse override

snapshot: Allow for post-parse override

Wire up the accessor functions necessary for the testsuite to install
an alternative post-parse handler from normal drivers. I could have
modified the signature for virDomainXMLOptionNew() to take another
parameter, but thought it was easier to add a new set function rather
than chase down all existing callers. Until code actually sets the
override, there is no change in behavior.

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