graphic: 0.911
device: 0.788
semantic: 0.585
files: 0.542
PID: 0.524
performance: 0.499
network: 0.451
other: 0.303
socket: 0.300
vnc: 0.291
permissions: 0.274
KVM: 0.268
boot: 0.257
debug: 0.227

qemu-img does not accept backing image file path, only file name
Description of problem:
In `qemu-img create ... -b <backing_image> ... <snapshot_image>`, <backing_image> cannot be a file path, but must be a file name. <backing_image> and <snapshot_image> are forced to be in the same directory for the command to work.
Steps to reproduce:
```
$ mkdir test
$ qemu-img create -f qcow2 test/a.img 1G
...
$ qemu-img create -f qcow2 -b test/a.img -F qcow2 test/a.img.snap
qemu-img: test/a.img.snap: Could not open 'test/test/a.img': No such file or directory
Could not open backing image.
$ qemu-img create -f qcow2 -b a.img -F qcow2 test/a.img.snap
...
$ ls test
a.img  a.img.snap
```