qemu-iotests: add test for pausing a streaming operation

These check that a paused streaming job does not advance its offset.

Sometimes the new test fails; the map is different between the source
and the destination of the streaming because qemu-io does not always
pack adjacent clusters that have the same allocated/unallocated state.
However, this also happens with the existing test_stream testcase, and
is better fixed in qemu-io.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
