qemu: Fix capstan run with relative image path

Pekka found this:

[penberg@localhost osv]$ capstan run build/release/usr.img
Created instance: i1396455503
qemu-system-x86_64: -drive
file=/home/penberg/.capstan/instances/qemu/i1396455503/disk.qcow2,if=none,id=hd0,aio=native,cache=none:
could not open disk image
/home/penberg/.capstan/instances/qemu/i1396455503/disk.qcow2: No such file or directory
exit status 1

The issue is that the image is a relative path so would be the
backing_file.

Fix it by making the backing_file a absolute path.

Signed-off-by: Asias He <asias@cloudius-systems.com>