xen-netfront, xen-netback: Use correct minimum MTU values

RFC791 specifies the minimum MTU to be 68, while xen-net{front|back}
drivers use a minimum value of 0.

When set MTU to 0~67 with xen_net{front|back} driver, the network
will become unreachable immediately, the guest can no longer be pinged.

xen_net{front|back} should not allow the user to set this value which causes
network problems.

Reported-by: Chen Shi <cheshi@redhat.com>
Signed-off-by: Mohammed Gamal <mgamal@redhat.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>