1. A computer-implemented method for managing virtual machine migration, the computer-implemented method comprising: under control of a hardware computing device configured with specific computer executable instructions, obtaining a first command to initiate migration of a virtual machine instance hosted on a first physical computing device to a second physical computing device, wherein the first command includes criteria for causing the hardware computing device to detect one or more migration events and specifies actions in response to detection of the one or more migration events; initiating a first phase of migration by copying contents of memory assigned to the virtual machine instance to the second physical computing device; and upon detecting a first migration event in accordance with the criteria included in the first command, wherein the first migration event corresponds to a determination to transition into a second phase of migration where at least the virtual machine is stopped and active memory is copied to the second physical computing device: delaying entering the second phase of migration; and transmitting a request for a second command to continue the migration of the virtual machine instance.