kvm tools: dynamically allocate p9 fids

kvm tools: dynamically allocate p9 fids

This removes the limit for p9 fids and the huge fid array that came along with
it. Instead, it dynamically allocates fids and stores them in a rb-tree.

This is useful when the guest needs a lot of fids, such as when stress
testing guests.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>