vz: fix boot check to use new disk id

vz: fix boot check to use new disk id

Current implementation does not detect all incompatible configurations.
For example if we have in vzsdk bootorder "cdrom1, cdrom0" (that is
"hdb, hda" in case of ide cdroms) and cdroms do not have disk
images inserted. In this case boot order check code fails to
distiguish them at all as for both PrlVmDev_GetFriendlyName gives "".
Well the consequences are only missing warnings but as
we just have introduced all the necessary tools to face the problem -
let's fix it.

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