Add hard build dependency on glib

Add hard build dependency on glib

GLib is an extremely common library that has a portable thread implementation
along with tons of other goodies.

GLib and GObject have a fantastic amount of infrastructure we can leverage in
QEMU including an object oriented programming infrastructure.

Short term, it has a very nice thread pool implementation that we could leverage
in something like virtio-9p.  It also has a test harness implementation that
this series will use.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@gmail.com>