Because XENMEM_maximum_reservation is rumored to change meaning in
the future, this patch adds a third amount-of-memory operation to
HYPERVISOR_memory_op() - XENMEM_maximum_memory.

This should make it possible to simply specify the maximum amount
of memory a guest will have in /etc/xen/<guest> instead of having
to pass it on the kernel commandline.

A next step would be to split out the maxmem values into two
different ones in the guest config file and the tools, but I'm
not quite sure what name to use there...

Signed-off-by: Rik van Riel <riel@redhat.com>

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