test_driver: Fix testDomainSetMemoryFlags' behaviour on config change

test_driver: Fix testDomainSetMemoryFlags' behaviour on config change

When commit 6ac402c456a added the API whenever VIR_DOMAIN_MEM_MAXIMUM
was passed the code always checked whether the domain was active and
therefore failed with an error even though only a config change was
requested. Fix the issue by replacing virDomainObjGetOneDef with
virDomainObjGetOneDefState which tells us what definition we're
performing the change on.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>