Clarify semantics of virDomainMigrate{,ToURI}2

Clarify semantics of virDomainMigrate{,ToURI}2

Commit 5d784bd6d7b19314b0908aec6b46bfe377aeba42 was a nice attempt to
clarify the semantics by requiring domain name from dxml to either match
original name or dname. However, setting dxml domain name to dname
doesn't really work since destination host needs to know the original
domain name to be able to use it in migration cookies. This patch
requires domain name in dxml to match the original domain name. The
change should be safe and backward compatible since migration would fail
just a bit later in the process.