kvm: x86: Move IA32_XSS to kvm_{get,set}_msr_common

Hoist support for RDMSR/WRMSR of IA32_XSS from vmx into common code so
that it can be used for svm as well.

Right now, kvm only allows the guest IA32_XSS to be zero,
so the guest's usage of XSAVES will be exactly the same as XSAVEC.

Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Change-Id: Ie4b0f777d71e428fbee6e82071ac2d7618e9bb40
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>