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

share/examples/bhyve/vmrun.sh

@@ -152,7 +152,7 @@ make_and_check_diskdev()
 {
     local virtio_diskdev="$1"
     # Create the virtio diskdev file if needed
-    if [ ! -f ${virtio_diskdev} ]; then
+    if [ ! -e ${virtio_diskdev} ]; then
 	    echo "virtio disk device file \"${virtio_diskdev}\" does not exist."
 	    echo "Creating it ..."
 	    truncate -s 8G ${virtio_diskdev} > /dev/null