KVM: MSI: Swap payload to native endianness

The usual MSI injection mechanism writes msi.data into memory using an
le32 wrapper. So on big endian guests, this swaps msg.data into the
expected byte order.

For irqfd however, we don't swap the payload right now, rendering
in-kernel MPIC emulation broken on PowerPC.

Swap msg.data to the correct endianness whenever we touch it.

Signed-off-by: Alexander Graf <agraf@suse.de>
