block: Propagate AioContext change to all children

block: Propagate AioContext change to all children

Instead of propagating any change of a BDS's AioContext only to its file
and backing children and letting driver-specific code do the rest, just
propagate it to all and drop the thus superfluous implementations of
bdrv_{at,de}tach_aio_context() in Quorum, blkverify and VMDK.

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Reviewed-by: Alberto Garcia <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>