Fix wrong used ProcessExecutionError exception

This class has been moved to nova.openstack.common.processutils,
but a wrong usage is exists in nova.virt.libvirt.volume,
correct here.

Closes-bug: #1308839
Change-Id: I76f99b63dc5097b462dcff6ff63cbbb13d7580fb
Merge "Fix wrong used ProcessExecutionError exception"