KVM: x86/mmu: Track shadow MMIO value on a per-VM basis

TDX will use a different shadow PTE entry value for MMIO from VMX.  Add a
member to kvm_arch and track value for MMIO per-VM instead of a global
variable.  By using the per-VM EPT entry value for MMIO, the existing VMX
logic is kept working.  Introduce a separate setter function so that guest
TD can use a different value later.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Message-Id: <229a18434e5d83f45b1fcd7bf1544d79db1becb6.1705965635.git.isaku.yamahata@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>