vfio: don't ignore return value of migrate_add_blocker

vfio: don't ignore return value of migrate_add_blocker

When an error occurs in migrate_add_blocker() it sets a
negative return value and uses error pointer we pass in.
Instead of just looking at the error pointer check for a negative return
value and avoid a coverity error because the return value is
set but never used. This fixes CID 1407219.

Reported-by: Coverity (CID 1407219)
Fixes: f045a0104c8c ("vfio: unplug failover primary device before migration")
Signed-off-by: Jens Freimann <jfreimann@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>