snapshot: Add internal option to validate XML against schema

Similar to VIR_DOMAIN_DEF_PARSE_VALIDATE_SCHEMA; the next patch will
put it to use with a counterpart public API flag.

No need to change qemudomainsnapshotxml2xmltest to use the flag, since
the testsuite already has a separate virschematest that does the same.

Signed-off-by: Eric Blake <eblake@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>