libxl: Free migration cookie

libxl: Free migration cookie

Commit id 'f9edcfa4' added cookie manipulation for libxl; however, some
cookie crumb cleanup was missed. Found by Coverity.

In libxlDomainMigrationBegin, the cookie is allocated and baked; however,
the mig ingredients weren't cleaned up.

In libxlDomainMigrationPrepare, when the 'mig' cookie is added to the
args, set the 'mig = NULL'; otherwise, other failure paths between when
the code ate the cookie data and when it was added to args would fail
to clean up the crumbs.

Signed-off-by: John Ferlan <jferlan@redhat.com>