migration: Add migration prefix to functions in target.c

The functions in target.c are not static, yet they don't have a proper
migration prefix. Add such prefix.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>