[virtio-9p] Add infrastructure to support glib threads and coroutines.

[virtio-9p] Add infrastructure to support glib threads and coroutines.

This patch is originally made by Arun Bharadwaj for glib support.
Later Harsh Prateek Bora added coroutines support.
This version implemented with suggestions from
Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>.

Signed-off-by: Arun R Bharadwaj <arun@linux.vnet.ibm.com>
Signed-off-by: Harsh Prateek Bora <harsh@linux.vnet.ibm.com>
Signed-off-by: Venkateswararao Jujjuri "<jvrao@linux.vnet.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>