xl: Some small fixes

xl: Some small fixes

- When use mem-set, I got suspicious error output:
  # xl mem-set 1 256g
  setting domid 1 memory to : 268435456
  [0] libxl.c:2535:libxl_set_memory_target: memory_dynamic_max must be
  less than or equal to memory_static_max
  : Success
- String generated by strdup() should be freed
- When using 'xl help', mem-max and mem-set's output is not as intend,
  and it also breaks bash completion, fix it.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>