Fix the bug of OSError when convert image

Fix the bug of OSError when convert image

When I try to convert a image use image_utils.convert_image() method,
an error occurred like this :
'''
OSError: [Errno 2] No such file or directory:
'sheepdog:10.133.17.61:7000:volume-a0a70f9b-a50e-4369-885f-c41a894c9fe5'
'''
The reason is that in some cluster storage systems, like
ceph/sheepdog, QEMU can access an image directly via their private
protocol, and there’s no need to map an image as a block device on
the host. In this case, the qemu-img convert command may like:

    #qemu-img convert -O raw sheepdog:Ip:port:image_name temp_file
    #qemu-img convert -O raw rbd:pool_name/image_name temp_file

The source path may be 'sheepdog:Ip:port:image_name' or
'rbd:pool_name/image_name', it doesn't exist in OS. So, when it runs
the os.stat(source) in image_utils.convert_image(source,dest,out_format)
method, an OSError would be raised.

We can use qemu_img_info method instead to resolve this problem, because
the 'qemu-img info' command can always get the image size info which has
support qemu-img tool. Here we capture a ValueError just in case, but it
only need to give a warning message, because the image has been successfully
converted.

Change-Id: I5fd1e51840972a67053b85a76f8e001fa8148ad7
Closes-Bug: #1514442