arm/arm64: KVM: avoid unnecessary guest register mangling on MMIO read

Currently we mangle the endianness of the guest's register even on an
MMIO _read_, where it is completely useless, because we will not use
the value of that register.
Rework the io_mem_abort() function to clearly separate between reads
and writes and only do the endianness mangling on MMIO writes.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>