xen/arm64: Correctly align VFP regs

xen/arm64: Correctly align VFP regs

On arm64, VFP instructions requires vfpregs to be 128-byte aligned.

By chance, the field is already correctly aligned. In the case if someone
decides to add a new field before, Xen will receive a data abort as soon as
it saves/restores VFP.

We are safe on arm32 as the only constraint is to be 32-byte aligned.

Reported-by: Chen Baozi <baozich@gmail.com>
Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>