qemu-iotests: skip 039 with ./check -nocache

When the qemu-io --nocache option is used the 039 test case cannot abort
QEMU at a point where the image is dirty.  Skip the test case.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
