KVM: VMX: rename vmx_shadow_fields.h to vmcs_shadow_fields.h

VMX specific files now reside in a dedicated subdirectory.  Drop the
"vmx" prefix, which is redundant, and add a "vmcs" prefix to clarify
that the file is referring to VMCS shadow fields.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>