The development of native wireless network virtualization implies introducing a new set of base station schedulers that considers the efficient allocation of wireless resources to different Service Providers (SPs) based on flexible Service Level Agreements (SLAs). In this paper we develop an efficient and fast centralized heuristic to allocate the radio resource blocks in multi-cell LTE networks. The scheme maximizes the network-wide sum rate while keeping track of the SLA of each SP expressed as a minimum bandwidth allocation in each cell. We also propose an iterative solution procedure for the non-convex power allocation problem based on DC programming. We find that the results of the heuristic are quite close to those of the iterative method. We also show that there is a significant rate reduction due to the service contracts. Finally, we find that even if the heuristic cannot meet all the requirements in one particular scheduling period, it does provide the required rate over a large number of periods.