target-ppc: Add hooks for handling tcg and kvm limitations

On target-ppc, our table of CPU types and features encodes the features as
found on the hardware, regardless of whether these features are actually
usable under TCG or KVM.  We already have cases where the information from
the cpu table must be fixed up to account for limitations in the emulation
method we're using.  e.g. TCG does not support the DFP and VSX instructions
and KVM needs different numbering of the CPUs in order to tell it the
correct thread to core mappings.

This patch cleans up these hacks to handle emulation limitations by
consolidating them into a pair of functions specifically for the purpose.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
[AF: Style and typo fixes, rename new functions and drop ppc_def_t arg]
Signed-off-by: Andreas Färber <afaerber@suse.de>
