KVM: Specify byte order for KVM_EXIT_MMIO

The KVM API documentation is not clear about the semantics of the data
field on the mmio struct on the kvm_run struct.

This has become problematic when supporting ARM guests on big-endian
host systems with guests of both endianness types, because it is unclear
how the data should be exported to user space.

This should not break with existing implementations as all supported
existing implementations of known user space applications (QEMU and
kvmtools for virtio) only support default endianness of the
architectures on the host side.

Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>