Clean up and fix hypercall macros in XenLinux: we must assume that

Clean up and fix hypercall macros in XenLinux: we must assume that
all register parameters are clobbered on return. Furthermore,
multicall arrays are clobbered on return from a multicall (only the
result field can be trusted!).

These restrictions are enforced by debug builds of Xen in the
following ways:
 1. On completion of a multicall, the call array is overwritten
    with garbage before writing in the result fields.
 2. On completion of a hypercall, all parameter registers are
    overwritten with garbage.

Signed-off-by: Keir Fraser <keir@xensource.com>