解决qcow2高版本兼容问题

'image' uses a qcow2 feature which is not supported by this qemu version: QCOW version 3

1.1 为新的QCOW V3 centos7/Fedora20 qemu-img支持格式，通过如下命令转化成低版本支持的QCOW2

qemu-img amend -f qcow2 -o compat=0.10 test.qcow2

qemu versions starting with 1.1 can use new qcow2 features which require
an incompatible on-disk format. Between version 1.1 and 1.6, they needed
to be specified explicitly during image creation, like this:

    qemu-img create -f qcow2 -o compat=1.1 test.qcow2 8G

Starting with qemu 1.7, compat=1.1 became the default, so that newly
created images can't be read by older qemu versions by default. If you
need to read them in older version, you now need to be explicit about
using the old format:

    qemu-img create -f qcow2 -o compat=0.10 test.qcow2 8G

With the same release, qemu 1.7, a new qemu-img subcommand was
introduced that allows converting between both versions, so you can
downgrade your existing v3 image to the format known by RHEL 6 like
this:

    qemu-img amend -f qcow2 -o compat=0.10 test.qcow2