qemu: Call qemuSetupHostdevCGroup later during hotplug

qemu: Call qemuSetupHostdevCGroup later during hotplug

https://bugzilla.redhat.com/show_bug.cgi?id=1025108

So far qemuSetupHostdevCGroup was called very early during hotplug, even
before we knew the device we were about to hotplug was actually
available. By calling the function later, we make sure QEMU won't be
allowed to access devices used by other domains.

Another important effect of this change is that hopluging USB devices
specified by vendor and product (but not by their USB address) works
again. This was broken since v1.0.5-171-g7d763ac, when the call to
qemuFindHostdevUSBDevice was moved after the call to
qemuSetupHostdevCGroup, which then used an uninitialized USB address.