libxl: Use virDomainLiveConfigHelperMethod for libxlDomainSetMemoryFlags
Flag expansion is the same as in virDomainObjUpdateModificationImpact
which virDomainLiveConfigHelperMethod calls internally. The difference
is merely in implementation. Note that VIR_DOMAIN_MEM_CONFIG is the
same as VIR_DOMAIN_AFFECT_CONFIG. Additionally, the called functions
will properly use flag OR and thus handle the VIR_DOMAIN_MEM_MAXIMUM case.
Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>