On ARM (32 bits and 64 bits), the double-word is 8-bytes aligned. This will
result on different structure from Xen and Linux repositories.

As Linux is using __packed__ attribute, it must have a 4-bytes padding before
each "id" field.

This change breaks guest block support with older kernel. IMHO, it's acceptable
because Xen on ARM is still on Tech Preview and the hypercall ABI is not yet
freezed.

Only one architecture (x86_32) doesn't have 64-bit ABI for the block interface.
Don't add padding if Linux is compiled for this architecture.
Signed-off-by: NJulien Grall <julien.grall@linaro.org>
Acked-by: NRoger Pau Monne <roger.pau@citrix.com>
Acked-by: NDavid Vrabel <david.vrabel@citrix.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Acked-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
[I had asked for confirmation that it did not break x86 and Ian went
beyound the call of duty to confirm it. Also a internal regression
bucket with 32/64 dom0 with 32/64 domU (PV and HVM) confirmed no
regressions. ABI changes are a drag..]
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>