gdbstub.c: fix GDB connection segfault caused by empty machines

This patch is to fix the segmentation fault caused by attaching
GDB to a QEMU instance initialized with "-M none" option.

The bug can be reproduced by

> ./qemu-system-x86_64 -M none -nographic -S -s

and attach a GDB to it by

> gdb -ex 'target remote :1234

The segmentation fault was originally caused by trying to read
the information about CPU when communicating with GDB. However,
it's impossible for any control flow to exist on an empty machine,
nor can CPU's be hot plugged to an empty machine later by QOM
commands. So I think simply disabling GDB connections on empty
machines makes sense.

Signed-off-by: Ziyue Yang <skiver.cloud.yzy@gmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>