kvm tools: treat uids and gids in stat structure properly

Upstream commit "9p: Modify the stat structures to use kuid_t and kgid_t"
has modified the type of uid and gid in the stat structure, which breaks
build for us.

This is a rather trivial conversion from u32 to kuid_t and kgid_t.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>