gdbstub: do not restart crashed guest

If a guest has crashed with an internal error or similar, detaching
gdb (or any other debugger action) should not restart it.

Cc: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-id: 1369912840-18577-1-git-send-email-pbonzini@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
