If we call virStreamFinish and virStreamAbort from 2 distinct
threads for example we can have access to freed memory.
Because when virStreamFinish finishes for example virStreamAbort
yet to be finished and it access virNetClientStreamPtr object
in stream->privateData.

Also it does not make sense to clear @driver field. After
stream is finished/aborted it is better to have appropriate
error message instead of "unsupported error".

This commit reverts [1] or virNetClientStreamPtr and
virStreamPtr will never be unrefed due to cyclic dependency.
Before this patch we don't have leaks because all execution
paths we call virStreamFinish or virStreamAbort.

[1] 8b6ffe40 : virNetClientStreamNew: Track origin stream
Signed-off-by: NNikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>