9pfs: rename virtio-9p-synth.{c,h} to 9p-synth.{c,h}

These two files are not virtio specific. Rename them to use generic
names.

Fix includes in various C files. Change define guards and comments
in header files.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
