Once qemu monitor reports migration has completed, we just closed our
end of the pipe and let migration tunnel die. This generated bogus error
in case we did so before the thread saw EOF on the pipe and migration
was aborted even though it was in fact successful.

With this patch we first wake up the tunnel thread and once it has read
all data from the pipe and finished the stream we close the
filedescriptor.

A small additional bonus of this patch is that real errors reported
inside qemuMigrationIOFunc are not overwritten by virStreamAbort any
more.