scripts/kvm/kvm_stat: Fix rlimit for unprivileged users

Setting the hard limit as a unprivileged user either returns an error
when it is higher than the current one or irreversibly sets it lower.

Therefore we leave the hardlimit untouched as long as we don't need to
raise it as this needs CAP_SYS_RESOURCE.

This gives admins the possibility to run the script as an unprivileged
user to increase security.

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Message-Id: <1452525484-32309-32-git-send-email-frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
