Previously Nova only checked the minimum required libvirt
version. It should also be checking for a minimum required
QEMU version.
This picks QEMU version 1.5.3, since that is the oldest
QEMU version that exists in the distros that include
the minimum libvirt version 1.2.1.
No decision is made about a "next" QEMU version at this
time.
Change-Id: Ie9413338a2422cc876f48c5c477eb3cd0ffb13ec