tools: Fix xm mem-max command.

tools: Fix xm mem-max command.

When I tested xm mem-max and xm list, I saw the following error
message by xm list.  Because xm mem-max always changes
"memory_static_max", I think that the cause of the error message
is it.  This patch fixes it.

# xm list --long PVdomain.1 | grep mem
    (maxmem 1024)
    (memory 1024)
    (shadow_memory 0)
# xm mem-max PVdomain.1 512
Error: (22, 'Invalid argument')
Usage: xm mem-max <Domain> <Mem>

Set the maximum amount reservation for a domain.
# xm list --long PVdomain.1 | grep mem
Error: memory_dynamic_max must be less than or equal to
memory_static_max

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>