Fix ordering of file open in virProcessGetNamespaces

Fix ordering of file open in virProcessGetNamespaces

virProcessGetNamespaces() opens files in /proc/XXX/ns/ which will
later be passed to setns(). We have to make sure that the file
descriptors in the array are in the correct order. In particular
the 'user' namespace must be first otherwise setns() may fail
for other namespaces.

The order has been taken from util-linux's sys-utils/nsenter.c

Also we must ignore EINVAL in setns() which occurs if the
namespace associated with the fd, matches the calling process'
current namespace.

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