KVM: VMX: Dedup code for adding MSR to VMCS's auto list

KVM: VMX: Dedup code for adding MSR to VMCS's auto list

Add a helper to add an MSR to a VMCS's "auto" list to deduplicate the code
in add_atomic_switch_msr(), and so that the functionality can be used in
the future for managing the MSR auto-store list.

No functional change intended.

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Tested-by: Manali Shukla <manali.shukla@amd.com>
Link: https://patch.msgid.link/20251206001720.468579-43-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>