arm/xen: Remove helpers which are PV specific

ARM guests are always HVM. The current implementation is assuming a 1:1
mapping which is only true for DOM0 and may not be at all in the future.

Furthermore, all the helpers but arbitrary_virt_to_machine are used in
x86 specific code (or only compiled for).

The helper arbitrary_virt_to_machine is only used in PV specific code.
Therefore we should never call the function.

Add a BUG() in this helper and drop all the others.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>