Currently there is no way to opt out of libvirt dropping POSIX
capabilities for qemu. This at least is a useful debugging tool, but
is also wanted by users (and distributors):

https://bugzilla.redhat.com/show_bug.cgi?id=559154
https://bugzilla.redhat.com/show_bug.cgi?id=573850

v2: Clarify qemu.conf comment, warn about security implications

v3: Add .aug changes