There would be too many extents that VMDK driver can't open all of them:

    005 0s ... - output mismatch (see 005.out.bad)
    --- 005.out     2013-12-24 09:27:27.608181030 +0800
    +++ 005.out.bad 2014-02-13 10:00:15.282184557 +0800
    @@ -4,10 +4,10 @@
     Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=5368709120000

     small read
    -read 4096/4096 bytes at offset 1024
    -4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
    +qemu-io: can't open device /tmp/qemu-iotests/t.vmdk: Could not open '/tmp/qemu-iotests/t-s1016.vmdk': Too many open files
    +no file open, try 'help open'

     small write
    -wrote 4096/4096 bytes at offset 8192
    -4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
    +qemu-io: can't open device /tmp/qemu-iotests/t.vmdk: Could not open '/tmp/qemu-iotests/t-s1016.vmdk': Too many open files
    +no file open, try 'help open'
     *** done

So disable the two subformats.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>