scripts/runqemu: show an error if /dev/net/tun is unusable

scripts/runqemu: show an error if /dev/net/tun is unusable

If /dev/net/tun is either not present or is not writable by the
user, then show an appropriate error message. (QEMU needs access to this
device in order to enable networking; it may be missing if it is not
enabled or loaded into the kernel, and some distributions such as CentOS
5.x set restrictive permissions upon it.)

(From OE-Core rev: a00b94900d437828f25debce1c30ffcc0bbf29e9)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>