migrate: Fix bounds check for migration parameters in migration.c

This patch fixes the out-of-bounds check of migration parameters in
qmp_migrate_set_parameters() for cpu-throttle-initial and
cpu-throttle-increment by adding a return statement for both as they
were broken since their introduction in 2.5 via commit 1626fee.
Due to the missing return statements, parameters were getting set to
out-of-bounds values despite the error.

Signed-off-by: Ashijeet Acharya <ashijeetacharya@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
