Convert Xen domain create/define/getxml/migration APIs to use virDomainDefPtr

Convert Xen domain create/define/getxml/migration APIs to use virDomainDefPtr

Introduce use of a virDomainDefPtr in the domain create, migrate,
getxml, & define APIs to simplify introduction of ACL security
checks. The virDomainPtr cannot be safely used, since the app
may have supplied mis-matching name/uuid/id fields. eg
the name points to domain X, while the uuid points to
domain Y. Resolving the virDomainPtr to a virDomainDefPtr
ensures a consistent name/uuid/id set.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>