KVM: x86: Open code shared_msr_update() in its only caller

Fold shared_msr_update() into its sole user to eliminate its pointless
bounds check, its godawful printk, its misleading comment (it's called
under a global lock), and its woefully inaccurate name.

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