hw/9pfs: Add synthetic file system support using 9p

hw/9pfs: Add synthetic file system support using 9p

This patch create a synthetic file system with mount tag
v_synth when -virtfs_synth command line option is specified
in qemu. The synthetic file system can be mounted in guest
using 9p using the below command line

mount -t 9p -oversion=9p2000.L,trans=virtio v_synth  <mountpint>

Synthetic file system enabled different qemu subsystem to register
callbacks for read and write events from guest. The subsystem
can create directories and files in the synthetic file system as show
in ex below

    qemu_v9fs_synth_mkdir(NULL, 0777, "test2", &node);
    qemu_v9fs_synth_add_file(node, 0777, "testfile",
                             my_test_read, NULL, NULL);

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