qemu-img: Initial progress printing support

This adds the basic infrastructure for supporting progress output
on the command line, as well as progress support for qemu-img commands
'rebase' and 'convert'.

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