powerpc: Cleanup KVM emulated load/store endian handling

Sometimes the KVM code on powerpc needs to emulate load or store
instructions from the guest, which can include both normal and byte
reversed forms.

We currently (AFAICT) handle this correctly, but some variable names are
very misleading.  In particular we use "is_bigendian" in several places to
actually mean "is the IO the same endian as the host", but we now support
little-endian powerpc hosts.  This also ties into the misleadingly named
ld_le*() and st_le*() functions, which in fact always byteswap, even on
an LE host.

This patch cleans this up by renaming to more accurate "host_swabbed", and
uses the generic swab*() functions instead of the powerpc specific and
misleadingly named ld_le*() and st_le*() functions.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>