KVM: Really fix HV_X64_MSR_APIC_ASSIST_PAGE

Commit 0945d4b228 tried to fix the get_msr path for the
HV_X64_MSR_APIC_ASSIST_PAGE msr, but was poorly tested.  We should be
returning 0 if the read succeeded, and passing the value back to the
caller via the pdata out argument, not returning the value directly.

Signed-off-by: Mike Waychison <mikew@google.com>
Signed-off-by: Avi Kivity <avi@redhat.com>