1. A system, comprising: a plurality of compute nodes that together implement a provider network, wherein each of the plurality of compute nodes comprise at least one processor and a memory; at least some of the plurality of compute nodes implement respective virtualization hosts for a plurality of virtual compute instances implemented as part of the provider network; a control plane for the provider network, the control plane configured to: maintain a resource credit pool comprising a plurality of resource credits available to replenish individual resource credit balances for the plurality of virtual compute instances that are authorized to expend resource credits from the same resource credit pool of the plurality of virtual compute instances, wherein the plurality of resource credits are individually expendable to increase utilization of a physical computer resource for an individual one of the authorized virtual compute instances in excess of a utilization guarantee for the physical computer resource at a respective virtualization host for the individual virtual compute instance for a period of time granted by the resource credit until the period of time for the resource credit is expended; receive a resource credit request for a virtual compute instance to replenish the individual resource credit balance for the virtual compute instance; evaluate the resource credit request to identify the virtual compute instance as one of the authorized virtual compute instances; in response to the identification of the virtual compute instance as authorized to expend resource credits from the resource credit pool: determine a number of resource credits to add to the individual resource credit balance for the virtual compute instance; send a response indicating the number of resource credits to be added to the individual resource credit balance for the virtual compute instance; and remove the calculated number of resource credits from the resource credit pool.