x86/HVM: meet xentrace's expectations on emulation event data
According to the logic in hvm_mmio_assist_process(), 64 bits of data are
expected with 64-bit addresses, and 32 bits of data with 32-bit ones. I
don't think this is very reasonable, but I'm also not going to touch the
consumer side, the more that it is anyway not very helpful for the code
here to only ever supply 32 bits of data (despite the field being 64
bits wide, and having been even in the 32-bit days of Xen).