xen: make gdbsx support configurable

xen: make gdbsx support configurable

Gdbsx support in the hypervisor is rarely used and it is opening a
way for dom0 to modify the running hypervisor by very easy means.

Remove the possibility to read/write hypervisor memory, it was never
used by gdbsx.

Add a Kconfig option to control support of gdbsx. Default is on.

While at it correct a wrong comment in related code and remove dead
code.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>