x86/hyperv: Clarify comment on x2apic mode

x86/hyperv: Clarify comment on x2apic mode

The comment about Hyper-V accessors is unclear regarding their
potential use in x2apic mode, as is the associated commit message
in e211288b72f1.  Clarify that while the architectural and
synthetic MSRs are equivalent in x2apic mode, the full set of xapic
accessors cannot be used because of register layout differences.

Fixes: e211288b72f1 ("x86/hyperv: Make vapic support x2apic mode")
Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/1603723972-81303-1-git-send-email-mikelley@microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>