xen: arm: do not panic when failing to translate a guest address

xen: arm: do not panic when failing to translate a guest address

The gva_to_{par,ipa} functions currently panic if the guest address
cannot be translated. Often the input comes from the guest so
panicing the host is a bit harsh!

Change the API of those functions to allow them to return a failure
and plumb it through where it is used.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>