qemu_hotplug: consolidate all common detach code in qemuDomainDetachDeviceLive

Now that all the qemuDomainDetachPrep*() functions look nearly
identical at the end, we can put one copy of that identical code in
qemuDomainDetachDeviceLive() at the point after the individual prep
functions have been called, and remove the duplicated code from all
the prep functions. The code to locate the target "detach" device
based on the "match" device remains, as do all device-type-specific
validations.

Unfortunately there are a few things going on at once in this patch,
which makes it a bit more difficult to follow than the others; it was
just impossible to do the changes in stages and still have a
buildable/testable tree at each step.

The other changes of note:

* The individual prep functions no longer need their driver or async
  args, so those are removed, as are the local "ret" variables, since
  in all cases the functions just directly return -1 or 0.

* Some of the prep functions were checking for a valid alias and/or
  for attempts to detach a multifunction PCI device, but not all. In
  fact, both checks are valid (or at least harmless) for *all* device
  types, so they are removed from the prep functions, and done a
  single time in the common function.

  (any attempts to *create* an alias when there isn't one has been
  removed, since that is doomed to failure anyway; the only way the
  device wouldn't have an alias is if 1) the domain was created by
  calling virsh qemu-attach to attach an existing qemu process to
  libvirt, and 2) the qemu command that started said process used "old
  style" arguments for creating devices that didn't have any device
  ids. Even if we constructed a device id for one of these devices,
  qemu wouldn't recognize it in the device_del command anyway, so we
  may as well fail earlier with "device missing alias" rather than
  failing later with "couldn't delete device net0".)

* Only one type of device has shutdown code that must not be called
  until after *all* validation of the device is done (including
  checking for multifunction PCI and valid alias, which is done in the
  toplevel common code). For this reason, the Net function has been
  split in two, with the 2nd half (qemuDomainDetachShutdownNet())
  called from the common function, right before sending the delete
  command to qemu.
Signed-off-by: NLaine Stump <laine@laine.org>
ACKed-by: NPeter Krempa <pkrempa@redhat.com>