Add the fields to support setting tls-creds and tls-hostname during
a migration (either source or target). Modify the query migration
function to check for the presence and set the field for future
consumers to determine which of 3 conditions is being met (NULL,
present and set to "", or present and sent to something). These
correspond to qemu commit id '4af245dc3' which added support to
default the value to "" and allow setting (or resetting) to ""
in order to disable. This reset option allows libvirt to properly
use the tls-creds and tls-hostname parameters.

Modify code paths that either allocate or use stack space in order
to call qemuMigrationParamsClear or qemuMigrationParamsFree for cleanup.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>