In maintaining the libvirt, we are generally pretty conservative when it comes to dropping support for old platforms. We recognize that while we'd love it if everyone was always running cutting edge distros, many people in corporate environments don't have this luxury. As a result the upstream libvirt code aims to build on any OS platform that is from RHEL-5 vintage circa 2006. This also serves to make life easier for our RHEL maintenance streams, where we'll often want to backport patches to fix bugs / add features targeting fairly old QEMU versions.
With RHEL-7 having been available for quite a while, last week we took the decision that it is reasonable to ditch support for RHEL-5 platforms from new upstream libvirt releases. The first result of this decision is that we have increased our minimum required QEMU version from 0.8.1 (release May 2006), to 0.12.0 (released Dec 2009). The 0.12.0 release was chosen between RHEL-6 ships with a (heavily patched) 0.12.1 release, and we don't wish to drop RHEL-6 support as a platform.
There shouldn't be any serious impact on other distros, as they're all typically at least as new as RHEL-6 in what they ship.
Dropping support for QEMU older than 0.12.0 is quite beneficial to libvirt, as there was a major effort starting in 0.11.0 to introduce a new way of configuring devices in QEMU, so we can ditch lots of code for the old approach from our command line generator.