xen/arm: Don't call setup_virtual_regions multiple time

xen/arm: Don't call setup_virtual_regions multiple time

The commit 2aa925be84293b44ad587ed117184ace61b41dd6 "arm/x86: Use struct
virtual_region to do bug, symbol, and (x86) exception tables lookup."
has introduced virtual_region. The call to initialize those regions is
made in init_traps which is called during each CPU bring up.

This will result to register multiple time the same region and Xen crash
when an address is looked up.

This can be fixed by moving the call to setup_virtual_region directly in
start_xen.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reported-by: Chenxia Zhao <chenxiao.zhao@gmail.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>