xen/x86: Zero out .bss for PV guests

xen/x86: Zero out .bss for PV guests

ELF spec is unclear about whether .bss must me cleared by the loader.
Currently the domain builder does it when loading the guest but because
it is not (or rather may not be) guaranteed we should zero it out
explicitly.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
(cherry picked from commit 04b6b4a56884327c1648c517f1f46a2638f04c9d)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
OraBug: 23268939