kvm tools: Add support for 9p2000.u

This patch adds support for the UNIX extensions to 9p2000.

Supporting thses extensions allow us to transperantly mount UNIX directories
without missing features such as symlinks.

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>