MFC 273102:

MFC 273102:
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.

PR:		210410

git-svn-id: svn://svn.freebsd.org/base/stable/10@303925 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f