1. A method of migrating a virtualized computing instance between source and destination virtualized computing systems, comprising: executing a first migration workflow in the source virtualized computing system, where a host computer in the source virtualized computing system executing the virtualized computing instance is a source host in the first migration workflow and a first mobility agent in the source virtualized computing system simulates a destination host computer in the first migration workflow, wherein the first migration workflow includes steps to migrate the virtualized computing instance from the host computer in the source virtualized computing system to the simulated destination host computer; blocking the first migration workflow by the first mobility agent at a point where the first mobility agent is prepared to receive data from the source host in the first migration workflow; executing a second migration workflow in the destination virtualized computing system, where a second mobility agent in the destination virtualized computing system simulates a source host computer in the second migration workflow and a host computer in the destination virtualized computing system is a destination host in the second migration workflow, wherein the second migration workflow includes steps to migrate a shadow virtualized computing instance of the virtualized computing instance from the simulated source host computer to the host computer in the destination virtualized computing system; blocking the second migration workflow by the second mobility agent at a point where the second mobility agent is prepared to send data to the destination host in the second migration workflow; unblocking the first and second migration workflows after the first and second migration workflows are synchronized at a point where both the first and second migration workflows can be executed concurrently; and transferring, after the first and second migration workflows are unblocked, migration data including the virtualized computing instance between the first mobility agent and the second mobility agent over a network.