qemu: fix improper initialization of cgroupControllers bitmap

qemu: fix improper initialization of cgroupControllers bitmap

virQEMUDriverConfigNew() always initializes the bitmap in its
cgroupControllers member to -1 (i.e. all 1's).

Prior to commit a9331394, if qemu.conf had a line with
"cgroup_controllers", cgroupControllers would get reset to 0 before
going through a loop setting a bit for each named cgroup controller.
commit a9331394 left out the "reset to 0" part, so cgroupControllers
would always be -1; if you didn't want a controller included, there
was no longer a way to make that happen.

This was discovered by users who were using qemu commandline
passthrough to use the "input-linux" method of directing
keyboard/mouse input to a virtual machine:

 https://www.redhat.com/archives/vfio-users/2016-April/msg00105.html

Here's the first report I found of the problem encountered after
upgrading libvirt beyond v2.0.0:

 https://www.redhat.com/archives/vfio-users/2016-August/msg00053.html

Thanks to sL1pKn07 SpinFlo <sl1pkn07@gmail.com> for bringing the
problem up in IRC, and then taking the time to do a git bisect and
find the patch that started the problem.