migration: Unexport migration_is_active()

migration: Unexport migration_is_active()

After being removed from VFIO and dirty limit, migration_is_active() no
longer has any users outside the migration subsystem, and in fact, it's
only used in migration.c.

Unexport it and also relocate it so it can be made static.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Tested-by: Joao Martins <joao.m.martins@oracle.com>
Link: https://lore.kernel.org/r/20241218134022.21264-8-avihaih@nvidia.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>