KVM: x86: Split kvm_is_valid_cr4() and export only the non-vendor bits

Split the common x86 parts of kvm_is_valid_cr4(), i.e. the reserved bits
checks, into a separate helper, __kvm_is_valid_cr4(), and export only the
inner helper to vendor code in order to prevent nested VMX from calling
back into vmx_is_valid_cr4() via kvm_is_valid_cr4().

On SVM, this is a nop as SVM doesn't place any additional restrictions on
CR4.

On VMX, this is also currently a nop, but only because nested VMX is
missing checks on reserved CR4 bits for nested VM-Enter.  That bug will
be fixed in a future patch, and could simply use kvm_is_valid_cr4() as-is,
but nVMX has _another_ bug where VMXON emulation doesn't enforce VMX's
restrictions on CR0/CR4.  The cleanest and most intuitive way to fix the
VMXON bug is to use nested_host_cr{0,4}_valid().  If the CR4 variant
routes through kvm_is_valid_cr4(), using nested_host_cr4_valid() won't do
the right thing for the VMXON case as vmx_is_valid_cr4() enforces VMX's
restrictions if and only if the vCPU is post-VMXON.

Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220607213604.3346000-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>