i386: Never free migration blocker objects instead of sometimes

i386: Never free migration blocker objects instead of sometimes

invtsc_mig_blocker has static storage duration.  When a CPU with
certain features is initialized, and invtsc_mig_blocker is still null,
we add a migration blocker and store it in invtsc_mig_blocker.

The object is freed when migrate_add_blocker() fails, leaving
invtsc_mig_blocker dangling.  It is not freed on later failures.

Same for hv_passthrough_mig_blocker and hv_no_nonarch_cs_mig_blocker.

All failures are actually fatal, so whether we free or not doesn't
really matter, except as bad examples to be copied / imitated.

Clean this up in a minimal way: never free these blocker objects.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210720125408.387910-7-armbru@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>