Fix preemption of multicalls in compat mode.

Fix preemption of multicalls in compat mode.

mcs->call (struct multicall_entry) always needs to be translated into
mcs->compat_call (struct compat_multicall_entry) when a multicall is
preempted in compat mode. Previously this translation only occured for
those hypercalls which explicitly called hypercall_xlat_continuation()
which doesn't cover all hypercalls which could potentially be
preempted.

Change hypercall_xlat_continuation() to only translate only the
hypercall arguments themselves and not the multicall_entry
layout. Translate the layout for all hypercalls in in
compat_multicall() instead.

Signed-off-by: Ian Campbell <ian.campbell@xensource.com>