Support older versions of qemu-img

Older versions of qemu-img do not accept a '--output json' option
Update ChangeLog and version files for 0.7rc3
Merge branch 'release-0.7' into debian-release-0.7
Bump version to 0.7~rc3-1~wheezy

Signed-off-by: Nikos Skalkotos <skalkoto@grnet.gr>