xen/arm: mm: Change the return value of gvirt_to_maddr

xen/arm: mm: Change the return value of gvirt_to_maddr

Currently, gvirt_to_maddr return -EFAULT when the translation failed.
It might be useful to return the PAR_EL1 (Physical Address Register)
in such a case to get a better idea of the reason.

So modify the return value to use 0 on success or return the PAR on
failure.

The callers are modified to reflect the change of the return value.

Note that with the change in gvirt_to_maddr, ma needs to be initialized
to avoid GCC been confused (i.e value may be uninitialized) with the new
construction.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>