libvirt: Tolerate existing vm(s) with cdrom(s)

When there is an existing vm with cdrom (not started by Nova), we
should tolerate missing file attribute (just skip those)

Fixes LP# 1144756

Change-Id: I43d45872e2d7348801b2e54cd43651609676f372
(cherry picked from commit cb369dab7184e72f24e70554828a7c4ca754d268)
Merge "libvirt: Tolerate existing vm(s) with cdrom(s)" into stable/folsom