hyperv: Make it obvious that hypervInvokeMethod() consumes an argument

hyperv: Make it obvious that hypervInvokeMethod() consumes an argument

Upon invocation, hypervInvokeMethod() consumes passed @params
(the second argument) regardless whether success or failure is
released. However, it takes only simple pointer (which hides this
ownership transfer) and because of that it doesn't clear it.

Switch to double pointer and tweak the documentation.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Matt Coleman <matt@datto.com>