write_msr_on_cpu is a helper to write a value to an MSR on a particular
CPU. The write must not cause a #GP fault as it is executed from a
context that handles IPI functions.

Tested:
1) See succesor CR which adds k cpu rdmsr/wrmsr commands.

2) Unit tests (x64_platform_tests) on qemu tcg, qemu kvm x64.

ZX-3359

Change-Id: I700a1997d47494904eda823f611eec253b8dace3