init checks for the root device as -f and it does not work because it is a block device

I can see my root fs as /dev/sda2, but when I submit it via cmdline in openvmm, the init does not recognize it, I think because the check should be -b and not -f:

```
if [ ! -f "$ROOT" ]; then
echo "Root device '$ROOT' does not exist. Dropping to a shell."
sh
fi
```

Log:

```
Root device '/dev/sda2' does not exist. Dropping to a shell.
sh: can't access tty; job control turned off
~ # ls -la /dev/sda*
brw------- 1 root root 8, 0 Jan 1 1970 /dev/sda
brw------- 1 root root 8, 1 Jan 1 1970 /dev/sda1
brw------- 1 root root 8, 2 Jan 1 1970 /dev/sda2

~ # [ ! -f "/dev/sda2" ]; echo $?
0
~ # [ ! -b "/dev/sda2" ]; echo $?
1
```

Start in the init script at the ROOT validation shown in the issue, focusing on how the root device is supplied from the openvmm command line. Reproduce with /dev/sda2 and confirm the check recognizes the block device instead of dropping to a shell. Done means valid root devices pass this check while missing devices still trigger the existing message.

Written by the indexing model from the issue text.