virtio-9p: Add a virtio 9p device to qemu

This patch doesn't implement the 9p protocol handling
code. It adds a simple device which dump the protocol data.

[jvrao@linux.vnet.ibm.com: Little-Endian to host format conversion]
[aneesh.kumar@linux.vnet.ibm.com: Multiple-mounts support]

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>