virSetUIDGID: Don't leak supplementary groups

virSetUIDGID: Don't leak supplementary groups

The LXC driver uses virSetUIDGID() to become UID/GID 0.
It passes an empty groups list to virSetUIDGID()
to get rid of all supplementary groups from the host side.
But virSetUIDGID() calls setgroups() only if the supplied list
is larger than 0.
This leads to a container root with unrelated supplementary groups.
In most cases this issue is unoticed as libvirtd runs as UID/GID 0
without any supplementary groups.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>