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
```