libxl_migration: Resolve Coverity RESOURCE_LEAK

libxl_migration: Resolve Coverity RESOURCE_LEAK

In libxlDomainMigrationPrepare() if the uri_in is false, then
'hostname' is allocated and used "generically" in the routine,
but not freed.  Conversely, if uri_in is true, then a uri is
allocated and hostname is set to the uri->hostname value and
likewise generically used.

At function exit, hostname wasn't free'd in the !uri_in path,
so that was added.  To just make it clearer on usage the else
path became the call to virURIFree() although I suppose technically
it didn't have to since it would be a call using (NULL)