KVM: x86: Mark vmcs12 pages as dirty if and only if they're mapped

KVM: x86: Mark vmcs12 pages as dirty if and only if they're mapped

Mark vmcs12 pages as dirty (in KVM's dirty log bitmap) if and only if the
page is mapped, i.e. if the page is actually "active" in vmcs02.  For some
pages, KVM simply disables the associated VMCS control if the vmcs12 page
is unreachable, i.e. it's possible for nested VM-Enter to succeed with a
"bad" vmcs12 page.

Link: https://patch.msgid.link/20251121223444.355422-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>