snapshot: test domainsnapshot indentation

snapshot: test domainsnapshot indentation

Add a test for the simple parts of my indentation changes, and
fix the fallout.

* tests/domainsnapshotxml2xmltest.c: New test.
* tests/Makefile.am (domainsnapshotxml2xmltest_SOURCES): Build it.
* src/conf/domain_conf.c (virDomainSnapshotDefFormat): Avoid NULL
deref, match documented order.
* src/conf/domain_conf.h (virDomainSnapshotDefFormat): Add const.
* tests/domainsnapshotxml2xmlout/all_parameters.xml: Tweak output.
* tests/domainsnapshotxml2xmlout/disk_snapshot.xml: Likewise.
* tests/domainsnapshotxml2xmlout/full_domain.xml: Likewise.
* .gitignore: Exempt new binary.