migrate: Fix cpu-throttle-increment regression in HMP

Commit 69ef1f36 accidentally broke migrate_set_parameter's ability
to set the cpu-throttle-increment to anything other than the
default, because it forgot to parse the user's string into an
integer.

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>