qemu_hotplug: standardize the names/args/calling of qemuDomainDetach*()

Most of these functions will soon contain only some setup for
detaching the device, not the detach code proper (since that code is
identical for these devices). Their device specific functions are all
being renamed to qemuDomainDetachPrep*(), where * is the
name of that device's data member in the virDomainDeviceDef
object.

Since there will be other code in qemuDomainDetachDeviceLive() after
the calls to qemuDomainDetachPrep*() that could still fail, we no
longer directly set "ret" with the return code from
qemuDomainDetachPrep*() functions, but simply return -1 on
failure, and wait until the end of qemuDomainDetachDeviceLive() to set
ret = 0.

Along with the rename, qemuDomainDetachPrep*() functions are also
given similar arglists, including an arg called "match" that points to
the proto-object of the device we want to delete, and another arg
"detach" that is used to return a pointer to the actual object that
will be (for now *has been*) detached. To make sure these new args
aren't confused with existing local pointers that sometimes had the
same name (detach), the local pointer to the device is now named after
the device type ("controller", "disk", etc). These point to the same
place as (*detach)->data.blah, it's just easier on the eyes to have,
e.g., "disk->dst" rather than "(*detach)->data.disk-dst".
Signed-off-by: NLaine Stump <laine@laine.org>
ACKed-by: NPeter Krempa <pkrempa@redhat.com>