blockresize: Skip the qed test if not supported.

1) Skip the qed test for newer libvirt who don't support this format.

2) Fix raw checking math to fix an error.

Signed-off-by: Hao Liu <hliu@redhat.com>
@rbian , When dealing with raw type image, qemu-img use disk sector to measure virtual size of a disk. There is a round up to 512 bytes when getting image info. So there is a chance that what we get is larger than expected value. Here is an example:
```
# dd if=/dev/zero of=test.raw bs=1000000 count=1
1+0 records in
1+0 records out
1000000 bytes (1.0 MB) copied, 0.000994072 s, 1.0 GB/s
# qemu-img info test.raw 
image: test.raw
file format: raw
virtual size: 977K (1000448 bytes)
disk size: 980K
```