iotests: check whitelisted formats

iotests: check whitelisted formats

Some test cases require specific formats. The method decorator
skip_if_unsupported() checks if requested formats are whitelisted.
The test #139 was selected for a sample output, after running
$ ./check -qcow2 131-140

137 3s ...
138 0s ...
139 2s ...
    [case not run] testBlkDebug (__main__.TestBlockdevDel): formats ['blkdebug'] are not whitelisted
    [case not run] testBlkVerify (__main__.TestBlockdevDel): formats ['blkverify'] are not whitelisted
    [case not run] testQuorum (__main__.TestBlockdevDel): formats ['quorum'] are not whitelisted
140 0s ...
Not run: 131 135 136
Some cases not run in: 139
Passed all 7 tests

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>