KVM: selftests: Add vcpu_arch_put_guest() to do writes from guest code

Introduce a macro, vcpu_arch_put_guest(), for "putting" values to memory
from guest code in "interesting" situations, e.g. when writing memory that
is being dirty logged.  Structure the macro so that arch code can provide
a custom implementation, e.g. x86 will use the macro to force emulation of
the access.

Use the helper in dirty_log_test, which is of particular interest (see
above), and in xen_shinfo_test, which isn't all that interesting, but
provides a second usage of the macro with a different size operand
(uint8_t versus uint64_t), i.e. to help verify that the macro works for
more than just 64-bit values.

Use "put" as the verb to align with the kernel's {get,put}_user()
terminology.

Link: https://lore.kernel.org/r/20240314185459.2439072-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/include/kvm_util_base.h

@@ -609,6 +609,9 @@ void *addr_gva2hva(struct kvm_vm *vm, vm_vaddr_t gva);
 vm_paddr_t addr_hva2gpa(struct kvm_vm *vm, void *hva);
 void *addr_gpa2alias(struct kvm_vm *vm, vm_paddr_t gpa);
 
+#ifndef vcpu_arch_put_guest
+#define vcpu_arch_put_guest(mem, val) do { (mem) = (val); } while (0)
+#endif
 
 static inline vm_paddr_t vm_untag_gpa(struct kvm_vm *vm, vm_paddr_t gpa)
 {