vz: fix crash when parsing unexpected disk configuration

vz: fix crash when parsing unexpected disk configuration

As it turned out PrlVmDev_GetStackIndex can return negative values
without reporting an error, which is incorrect but nevertheless.
After that we feed this negative index to virIndexToDiskName,
which in turn returns NULL and we set it to virDomainDiskDef.dst.
Using virDiskNameToBusDeviceIndex with a virDomainDiskDef structure
which has NULL dst field crashes.
Fix this by returning an error in prlsdkGetDiskId in such cases.

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