block: add throttle block filter driver

block: add throttle block filter driver

block/throttle.c uses existing I/O throttle infrastructure inside a
block filter driver. I/O operations are intercepted in the filter's
read/write coroutines, and referred to block/throttle-groups.c

The driver can be used with the syntax
-drive driver=throttle,file.filename=foo.qcow2,throttle-group=bar

which registers the throttle filter node with the ThrottleGroup 'bar'. The
given group must be created beforehand with object-add or -object.

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