vl: revert behaviour for -display none

vl: revert behaviour for -display none

Commit 1bec1cc0d ("ui/console: allow to override the default VC") changed
the behaviour of the "-display none" option, so that it now creates a
QEMU monitor on the terminal. "-display none" should not be tangled up
with whether we create a monitor or a serial terminal; it should purely
and only disable the graphical window. Changing its behaviour like this
breaks command lines which, for example, use semihosting for their
output and don't want a graphical window, as they now get a monitor they
never asked for.

It also breaks the command line we document for Xen in
docs/system/i386/xen.html:

 $ ./qemu-system-x86_64 --accel kvm,xen-version=0x40011,kernel-irqchip=split \
    -display none -chardev stdio,mux=on,id=char0,signal=off -mon char0 \
    -device xen-console,chardev=char0  -drive file=${GUEST_IMAGE},if=xen

qemu-system-x86_64: cannot use stdio by multiple character devices
qemu-system-x86_64: could not connect serial device to character backend
'stdio'

When qemu is compiled without PIXMAN, by default the serials aren't
muxed with the monitor anymore on stdio. The serials are redirected to
"null" instead, and the monitor isn't set up.

Fixes: commit 1bec1cc0d ("ui/console: allow to override the default VC")
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>