Part of a series of cleanups to use new accessor methods.

Several places in domain_conf.c still open-code raw field access,
but that code will be touched later with the diskDef struct split
so I'm avoiding churn here.

* src/conf/domain_audit.c (virDomainAuditStart): Use accessors.
* src/conf/domain_conf.c (virDomainDiskIndexByName)
(virDomainDiskPathByName, virDomainDiskDefForeachPath)
(virDomainDiskSourceIsBlockType): Likewise.
* src/conf/snapshot_conf.c (virDomainSnapshotAlignDisks):
Likewise.
Signed-off-by: NEric Blake <eblake@redhat.com>