vz: delete domains when undefine is called

vz: delete domains when undefine is called

Currently vz driver unregisters domains when undefine is called,
which is wrong because it contradicts with expected behavior.
All vz domains are persistent, which means that when one is
defined a new bundle directory containing meta data is created.
Undefining domains in a way we do now leaves those directories
undeleted, which prevents subsequent define call for the same
domain xml. I.e. the following sequence define->undefine->define
doesn't work now.
The patch fixes the problem by calling PrlVm_Delete instead of
PrlVm_Unreg detaching all disks prior actually doing this to
prevent images deletion.

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