1. A computer-implemented method for migrating a virtual machine instance, comprising: under the control of one or more computer systems of a virtual computer system service, the one or more computer systems configured with executable instructions, receiving a set of preferences from a customer, the set of preferences including one or more hardware specifications for instantiating a virtual machine instance that indicates an optionally specifiable hardware preference for instantiating a virtual machine instance, the set of preferences received via an interface through which one or more requests for instantiation of a virtual machine instance from a plurality of virtual machine instance types can be submitted, one or more of the plurality of virtual machine instance types being capable of being requested with the optionally specifiable hardware preference; instantiating the virtual machine instance to result in an instantiated virtual machine instance in a first physical host computer system, the first physical host computer system comprising hardware that does not conform to the set of preferences by lacking the indicated optionally specifiable hardware preference at the time of instantiation of the virtual machine instance; generating for the instantiated virtual machine instance, a metadata flag that identifies that the instantiated virtual machine instance should be migrated to a physical host computer system that conforms to the set of preferences by comprising hardware that has the indicated optionally specifiable hardware preference if capacity within a conforming physical host computer system becomes available that comprises the indicated optionally specifiable hardware preference defined by the preferences; detecting, as a result of the metadata flag, available capacity within a second physical host computer system after instantiation of and during execution of the virtual machine instance, the second physical host computer system comprising hardware that conforms to the set of preferences by comprising the indicated optionally specifiable hardware preference defined by the preferences; and migrating, as a result of the metadata flag, the virtual machine instance from the first physical host computer system to the second physical host computer system partially due to having detected the available capacity within the second physical host computer system.