pc-dimm: assign and verify the "slot" property during pre_plug

pc-dimm: assign and verify the "slot" property during pre_plug

We can assign and verify the slot before realizing and trying to plug.
reading/writing the slot property should never fail, so let's reduce
error handling a bit by using &error_abort.

To do this during pre_plug, add and use (x86, ppc) pc_dimm_pre_plug().

Reviewed-by: David Gibson <[email protected]>
Reviewed-by: Igor Mammedov <[email protected]>
Reviewed-by: Eric Auger <[email protected]>
Signed-off-by: David Hildenbrand <[email protected]>
Message-Id: <20180801133444[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>