You can use virsh to modify a Virtual Machine's maximum memory:
virsh setmaxmem [domain-name | domain-id | domain-uuid] [count]
You must specify the [count] in kilobytes. Note that the new count cannot exceed the amount you specified when you created the Virtual Machine. Values lower than 64 MB probably won't work. The maximum memory doesn't affect the current use of the Virtual Machine (unless the new value is lower which should shrink memory usage).