qdev: HotplugHandler: rename unplug callback to unplug_request

'HotplugHandler.unplug' callback is currently used as async
call to issue unplug request from device that implements it.
Renaming 'unplug' callback to 'unplug_request' should help to
avoid confusion about what callback does and would allow to
introduce 'unplug' callback that would perform actual device
removal when hardware is ready for it.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
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Patch is prompted by reviewing https://patchwork.ozlabs.org/patch/383372/

Dedicated 'unplug' callback could be used by bus-less pc-dimm
device. It would allow to call HotplugHandler.unplug callback
from ACPI code when guest calls _EJ0 method and execute board
specific code (PCMachine) to unmap pc-dimm from guest's address
space and perform necessary cleanup.