kvm tools: check read permission before using the root partition of the host to boot VM

The commit fbe8d0f (kvm tools: Use the root partition of the host to boot the
guest machine) changed the default image for virtual machine to root partition
of the host machine. The patch adds a check to ensure appropriate permission
(a read permission) is available for kvm tool to use this partition.

Signed-off-by: Prasad Joshi <prasadjoshi124@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

kvm-run.c

@@ -264,6 +264,10 @@ static int root_device(char *dev, long *part)
 	/* verify the device path */
 	if (stat(dev, &st) < 0)
 		return -1;
+
+	if (access(dev, R_OK) < 0)
+		return -1;
+
 	return 0;
 }