https://bugzilla.redhat.com/show_bug.cgi?id=740899 documents that
if qemu uses aio=native for its disks, then it consumes 128 aio
requests per disk.  On a host with multiple guests, this can quickly
run out of kernel aio requests with the default aio-max-nr of
65536.  Kernel developers have confirmed that there is no up-front
cost to raising this limit (a larger limit merely implies that more
aio requests can be issued in parallel, which in turn will result
in more kernel memory allocation, only if the system really does use
that many requests).  Since the system default limit prevents 256
disks, which is well within libvirt's current scalability, this
patch installs a file to raise the limit and document it in case a
system administrator has further cause to tune the limit.  The
install only works on platforms new enough to source /etc/sysctl.d/*
alongside /etc/sysctl.conf (F14 and RHEL 6).

* daemon/libvirtd.sysctl: New file.
* daemon/Makefile.am (EXTRA_DIST): Ship it.
(install-init, uninstall-init): Install it.
* libvirt.spec.in (%files): Include it in rpm.