core: Text message fixing.

Text fix for a message returned from trying to move an export
domain to maintenance while a VM is exported to it.

Change-Id: I8ed79d6de27fbeb907116dfbaee863891858bbc6
Bug-Url: https://bugzilla.redhat.com/1167773
Signed-off-by: Amit Aviram <aaviram@redhat.com>