kvm tools: Introduce 'kvm setup' command

This patch implements 'kvm setup' command that can be used to setup a guest
filesystem that shares system libraries and binaries from host filesystem in
read-only mode.

You can setup a new shared rootfs guest with:

  ./kvm setup -n default

and launch it with:

  ./kvm run --9p /,hostfs -p "init=virt/init" -d ~/.kvm-tools/default/

We want to teach 'kvm run' to be able to launch guest filesystems by name in
the future. Furthermore, 'kvm run' should setup a 'default' filesystem and use
it by default unless the user specifies otherwise.

Cc: Asias He <asias.hejun@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>