iotests: chown LUKS device before qemu-io launches

On some distros, whenever you close a block device file
descriptor there is a udev rule that resets the file
permissions. This can race with the test script when
we run qemu-io multiple times against the same block
device. Occasionally the second qemu-io invocation
will find udev has reset the permissions causing failure.

Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170626123510.20134-6-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
