KVM: VMX: Move caching of MSR_IA32_XSS to hardware_setup()

[ Upstream commit 71d9409e20934e16f2d2ea88f0d1fb9851a7da3b ]

MSR_IA32_XSS has no relation to the VMCS whatsoever, it doesn't belong
in setup_vmcs_config() and its reference to host_xss prevents moving
setup_vmcs_config() to a dedicated file.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Stable-dep-of: a44b331614e6 ("KVM: x86/vmx: Do not skip segment attributes if unusable bit is set")
Signed-off-by: Sasha Levin <sashal@kernel.org>