block/throttle-groups.c: allocate RestartData on the heap

block/throttle-groups.c: allocate RestartData on the heap

RestartData is the opaque data of the throttle_group_restart_queue_entry
coroutine. By being stack allocated, it isn't available anymore if
aio_co_enter schedules the coroutine with a bottom half and runs after
throttle_group_restart_queue returns.

Cc: qemu-stable@nongnu.org
Signed-off-by: Manos Pitsidianakis <el13635@mail.ntua.gr>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>