qemu-iotests: Extend 041 for unbacked mirroring

Add a new test case in file 041 for mirroring unbacked images in
"absolute-paths" mode. This should work, if possible, but most
importantly, qemu should never crash.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
