Documentation: Warn against qemu-img on active image

Documentation: Warn against qemu-img on active image

People have repeatedly expected that you can do things like snapshotting
an image with qemu-img while a qemu instance is running. Maybe we need
to consider locking the files while they are in use, but having a
warning in the qemu-img manpage is doable for 1.2 and can't hurt anyway.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 48467328c6dccc9c7be6628ed0ef0810f83be2a9)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>