This adds basic support for domain migrations from one hypervisor to
another. Migration options, e.g., live, tunneled, compressed, etc..,
are specified by the constants described `constants.Migrate*`.
Two unknowns remain, Libvirt specifies `RemoteParameters` and `CookieIn`.
In testing both values are always set to 0 by `virsh` and the source
does not provide clear definitions of their purpose. For now, using
the same zero'd values used by `virsh` will be Good Enough.
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