x86: qemu_x86_64: workaround SMP issues in x86

We have some races causing random failures with this platform, set cpu
number to one while we investigate and fix the issue.

Related to zephyrproject-rtos#21317

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

boards/x86/qemu_x86/qemu_x86_64_defconfig

@@ -17,3 +17,4 @@ CONFIG_SMP=y
 CONFIG_MP_NUM_CPUS=2
 CONFIG_X86_MMU=y
 CONFIG_X86_VERY_EARLY_CONSOLE=y
+CONFIG_MP_NUM_CPUS=1