Revert "v2v: -i libvirtxml: Give an error if local disks are not readable (RHBZ#1140946)."

This commit breaks remote disks.  See:
https://bugzilla.redhat.com/show_bug.cgi?id=1141654

This reverts commit 73c1f1cf7e1c4aa4e70407feff49ba847f7ebb63.