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: [email protected]
Signed-off-by: Manos Pitsidianakis <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Alberto Garcia <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>