Revert "qdev: Use qdev_get_device_class() for -device <type>,help"

This reverts commit 31bed5509dfcbdfc293154ce81086a4dbd7a80b6.

The reverted commit changed qdev_device_help() to reject abstract
devices and devices that have cannot_instantiate_with_device_add_yet
set, to fix crash bugs like -device x86_64-cpu,help.

Rejecting abstract devices makes sense: they're purely internal, and
the implementation of the help feature can't cope with them.

Rejecting non-pluggable devices makes less sense: even though you
can't use them with -device, the help may still be useful elsewhere,
for instance with -global.  This is a regression: -device FOO,help
used to help even for FOO that aren't pluggable.

The previous two commits fixed the crash bug at a lower layer, so
reverting this one is now safe.  Fixes the -device FOO,help
regression, except for the broken devices marked
cannot_even_create_with_object_new_yet.  For those, the error message
is improved.

Example of a device where the regression is fixed:

    $ qemu-system-x86_64 -device PIIX4_PM,help
    PIIX4_PM.command_serr_enable=bool (on/off)
    PIIX4_PM.multifunction=bool (on/off)
    PIIX4_PM.rombar=uint32
    PIIX4_PM.romfile=str
    PIIX4_PM.addr=int32 (Slot and optional function number, example: 06.0 or 06)
    PIIX4_PM.memory-hotplug-support=bool
    PIIX4_PM.acpi-pci-hotplug-with-bridge-support=bool
    PIIX4_PM.s4_val=uint8
    PIIX4_PM.disable_s4=uint8
    PIIX4_PM.disable_s3=uint8
    PIIX4_PM.smb_io_base=uint32

Example of a device where it isn't fixed:

    $ qemu-system-x86_64 -device host-x86_64-cpu,help
    Can't list properties of device 'host-x86_64-cpu'

Both failed with "Parameter 'driver' expects pluggable device type"
before.

Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <1443689999-12182-11-git-send-email-armbru@redhat.com>
