libxl: fix "xl mem-set" regression from 0c029c4da2

libxl: fix "xl mem-set" regression from 0c029c4da2

Said commit ("libxl_set_memory_target: retain the same maxmem offset on
top of the current target") caused a regression for "xl mem-set"
against Dom0: While prior to creation of the first domain this works,
the first domain creation involving ballooning breaks. Due to "enforce"
not being set in the domain creation case, and due to Dom0's initial
->max_pages (in the hypervisor) being UINT_MAX, the calculation of
"memorykb" in the first "xl mem-set" adusting the target upwards
subsequent to domain creation and termination may cause an overflow,
resulting in Dom0's maximum getting to a very small value. This small
maximum will the make the subsequent setting of the PoD target fail.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>