x86: Out-of-bounds read in vRTC emulation

Accesses to the CMOS memory contents are done using an indirect IO port

pair. Therefore Xen needs to cache the guest chosen index, and one of

the usages of the index didn't take the necessary locking to avoid

concurrent changes. As a result, a guest could change the index after

it being checked, causing a subsequent out-of-bound read access to the

contents of an array.