Add generic drive hotplugging

The monitor command for hotplugging is in i386 specific code. This is just
plain wrong, as S390 just learned how to do hotplugging too and needs to
get drives for that.

So let's add a generic copy to generic code that handles drive_add in a
way that doesn't have pci dependencies. All pci specific code can then
be handled in a pci specific function.

Signed-off-by: Alexander Graf <agraf@suse.de>

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v1 -> v2:

  - align generic drive_add to pci specific one
  - rework to split between generic and pci code

v2 -> v3:

  - remove comment