qemuimg: support relative backing file in create

This patch adds the support for creating a qcow2 images with a relative
backing path. In some cases qemu-img requires to access the backing
image and being the path relative we need to execute qemu-img with the
proper cwd.

Change-Id: I5321ca35edd25c5f4972b0ef151ede55907be7b7
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26870
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>