qemu-tool: revert cpu_get_clock() abort(3)

qemu-tool: revert cpu_get_clock() abort(3)

Despite the fact that the qemu-tool environment has no guest running and
vm_clock therefore does not make sense, there is code that gets the
vm_clock time even in qemu-tool.  Therefore, revert the abort(3) call
and just return 0 like we used to.  This unbreaks qemu-img/qemu-io with
QED and Kevin has also expressed interest in this for qcow2.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>