vz: reuse edit config frame in for attach/detach functions

vz: reuse edit config frame in for attach/detach functions

Attach/detach functions for disk/net are quite trivial and
typically call a few functions in begin/end edit frame. Having
in mind update function too adding configuring for another
device (like graphics) will introduce 3 trivial functions more.
Let's replace current approach by attach/detach functions for
device.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Signed-off-by: Maxim Nestratov <mnestratov@virtuozzo.com>