um: change defconfig to stop spawning xterm

The default configuration dictates that um Linux will launch xterms
connected to various tty devices on the host.  It makes more sense to
connect to /dev/pts/N, so that the host can:

  $ screen /dev/pts/8

  or

  $ minicom /dev/pts/8

to start talking to the guest session.  Note that
con0 (CONFIG_CON_ZERO_CHAN) still defaults to "fd:0,fd:1", printing boot
messages on the same terminal.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Richard Weinberger <richard@nod.at>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>