ANDROID: pkvm: x86: Add support for protected guests on PKVM hypervisor

Add a new config PKVM_GUEST. Once enable this config, the kernel can run
on the pkvm as a protected VM. Now the guest just detect if it's running
as a pkvm guest, later will add more functionality.

Bug: 395299836
Test: Boot, verify cpus are de-privileged and run a minimal protected vm.

Change-Id: Ibe95f321395fa6175b6dc66f45f89db334939e5c
Signed-off-by: Shaoqin Huang <shaoqin.huang@intel.com>
Reviewed-by: Jason Chen CJ <jason.cj.chen@intel.com>
Signed-off-by: Vineeth Pillai <vineethrp@google.com>