x86/Xen: correct commentary and parameter naming of xen_exchange_memory()

x86/Xen: correct commentary and parameter naming of xen_exchange_memory()

As documented in comments in struct xen_memory_exchange, the input to the
hypercall is a set of MFNs which are to be removed from the domain, plus a
set of PFNs where the newly allocated MFNs are to appear. Present comment
and parameter naming don't correctly reflect that.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <7e0c8795-cc60-4b78-8601-6a999739467a@suse.com>