This fixes a bug in truncated response payload handling. Payloads
requiring more than one call to `Read()` were overwiting the previous
Read()'s buffer contents. Because we know the response payload size, we
can simply call `io.ReadAll()` rather than looping over calls to
`Read()`.
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.
