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.
This splits out the mock libvirt server for testing within
other packages. Constants from the main libvirt package
have been moved to a separate package, constants, for shared access.
