VFIO device assignment requires a cgroup ACL to be setup for access to
the /dev/vfio/nn "group" device for any devices that will be assigned
to a guest. In the case of a host device that is allocated from a
pool, it was being allocated during qemuBuildCommandLine(), which is
called by qemuProcessStart() *after* the all-encompassing
qemuSetupCgroup() was called, meaning that the standard Cgroup ACL
setup wasn't creating ACLs for these devices allocated from pools.

One possible solution was to manually add a single ACL down inside
qemuBuildCommandLine() when networkAllocateActualDevice() is called,
but that has two problems: 1) the function that adds the cgroup ACL
requires a virDomainObjPtr, which isn't available in
qemuBuildCommandLine(), and 2) we really shouldn't be doing network
device setup inside qemuBuildCommandLine() anyway.

Instead, I've created a new function called
qemuNetworkPrepareDevices() which is called just before
qemuPrepareHostDevices() during qemuProcessStart() (explanation of
ordering in the comments), i.e. well before the call to
qemuSetupCgroup(). To minimize code churn in a patch that will be
backported to 1.0.5-maint, qemuNetworkPrepareDevices only does
networkAllocateActualDevice() and the bare amount of setup required
for type='hostdev network devices, but it eventually should do *all*
device setup for guest network devices.

Note that some of the code that was previously needed in
qemuBuildCommandLine() is no longer required when
networkAllocateActualDevice() is called earlier:

 * qemuAssignDeviceHostdevAlias() is already done further down in
   qemuProcessStart().

 * qemuPrepareHostdevPCIDevices() is called by
   qemuPrepareHostDevices() which is called after
   qemuNetworkPrepareDevices() in qemuProcessStart().

As hinted above, this new function should be moved into a separate
qemu_network.c (or similarly named) file along with
qemuPhysIfaceConnect(), qemuNetworkIfaceConnect(), and
qemuOpenVhostNet(), and expanded to call those functions as well, then
the nnets loop in qemuBuildCommandLine() should be reduced to only
build the commandline string (which itself can be in a separate
qemuInterfaceBuilldCommandLine() function as suggested by
Michal). However, this will require storing away an array of tapfd and
vhostfd that are needed for the commandline, so I would rather do that
in a separate patch and leave this patch at the minimum to fix the
bug.