KVM: x86: Move nested_run_pending to kvm_vcpu_arch

KVM: x86: Move nested_run_pending to kvm_vcpu_arch

Move nested_run_pending field present in both svm_nested_state and
nested_vmx to the common kvm_vcpu_arch. This allows for common code to
use without plumbing it through per-vendor helpers.

nested_run_pending remains zero-initialized, as the entire kvm_vcpu
struct is, and all further accesses are done through vcpu->arch instead
of svm->nested or vmx->nested.

No functional change intended.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
[sean: expand the commend in the field declaration]
Link: https://patch.msgid.link/20260312234823.3120658-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>