Use '-e' to check if the virtio backing file has already been created.

Use '-e' to check if the virtio backing file has already been created.

The '-f' check works fine on a regular file but not if the backing file is
a device (e.g., /dev/md0). In this case it would print a misleading but
otherwise benign message about the backing file not being present.

Submitted by:	Marcus Reid (marcus@blazingdot.com)
Discussed with:	grehan