vz: fix vzCheckUnsupportedDisks format checks for cdroms

vz: fix vzCheckUnsupportedDisks format checks for cdroms

Current version of the function does not check format of cdroms at all.
At the same time prlsdkGetDiskInfo give hints that cdroms always
have format VIR_STORAGE_FILE_RAW. So fix vzCheckUnsupportedDisks.

About structure of checks. As we don't have means to store format
in SDK we always have only one format in every situation. So instead
of setting boolean let's get allowed format instead and finally compare
it to the requested. This structure of checks seems stable to me
because we have only one format in every situation.

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