qemu.tests: disable qemu_iotests for some image format on RHEL host

since qcow, qed, vmdk, vpc and vdi not support by qemu-kvm in RHEL so
filter out tests on these image format;

Signed-off-by: Xu Tian <xutian@redhat.com>
Merge pull request #690 from xutian/fix_qemu_iotest

disable qemu-iotest for some image format on RHEL host