1. A system that virtualizes a computer, the system comprising: a processor and a memory; a virtual-machine monitor executing on the processor; a virtual machine managed by the virtual-machine monitor, the virtual machine executing an instruction stream that includes two or more privilege-dependent instructions; and a virtualization-selection control that is invoked by the virtual-machine monitor following a trap and virtual-machine-exit transition which occurs when the processor attempts to execute a first privilege-dependent instruction and that selects one of multiple virtualization-support subsystems estimated to have a lowest performance overhead to process a sequence of instructions which follows the first privilege-dependent instruction in the instruction stream executed by the virtual machine, the sequence of instructions including one or more additional privilege-dependent instructions and any non privilege-dependent instructions temporally located between the privilege-dependent instructions, the multiple virtualization-support subsystems including a first virtualization-support subsystem that emulates execution of the first privilege-dependent instruction, then executes a virtual-machine-entry transition, and that is again invoked to emulate each of the one or more additional privilege-dependent instructions, and a second virtualization-support subsystem that emulates execution of the first privilege-dependent instruction as well as the sequence of instructions prior to executing a virtual-machine-entry transition.