vz: handle sourceless cdroms

vz: handle sourceless cdroms

SDK handles empty cdroms all right. We just need to
pass "" instead of NULL (not setting is good too).

However we can get problems here. Disk detaching treats source
as ids. Fortunately disk detaching is not supported for cdroms
yet and for hard disks we can not get empty source - this is prohibitited
by xml parsing code.

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