virDomainSetMemory to set up the target memory use for a domain
* src/driver.h src/libvirt.c src/xen_internal.c src/xend_internal.[ch]
  src/xs_internal.[ch]: added a new entry point in the drivers for this,
  also fixed the xen store entry, as changing
  /local/domain/$$/memory/target affects the target memory not the max,
  apparently max is not visible at the xenstore level (or I missed it)
Daniel