io: some fixes to handling of /dev/null when running commands

The /dev/null file handle was leaked in a couple of places.
There is also the possibility that both readfd and writefd
point to the same /dev/null file handle, so care must be
taken not to close the same file handle twice.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
