QAPI will require glib/python, but for now the guest agent is the only
user. For now, make these dependencies an explicit guest agent one, and
give users the option to disable it if need be.

Once QAPI is adopted in core QEMU code, we would basically revert this
patch.
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>