kvm: nVMX: Add support for "VMWRITE to any supported field"

Add support for "VMWRITE to any supported field in the VMCS" and
enable this feature by default in L1's IA32_VMX_MISC MSR. If userspace
clears the VMX capability bit, the old behavior will be restored.

Note that this feature is a prerequisite for kvm in L1 to use VMCS
shadowing, once that feature is available.

Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>