Move debug exception handling out of cpu_exec

Move debug exception handling out of cpu_exec

To prepare splitting up KVM and TCG CPU entry/exit, move the debug
exception into cpus.c and invoke cpu_handle_debug_exception on return
from qemu_cpu_exec.

This also allows to clean up the debug request signaling: We can assign
the job of informing main-loop to qemu_system_debug_request and stop the
calling cpu directly in cpu_handle_debug_exception. That means a debug
stop will now only be signaled via debug_requested and not additionally
via vmstop_requested.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>