xen: add upstream post-4.9.0 security fix for XSA-245

Fixes XA-245: ARM: Some memory not scrubbed at boot

https://xenbits.xenproject.org/xsa/advisory-245.html



Notice: Not applying XSA-237..244 as they are x86 only and have patch file
name conflicts between 2017.02.x and master.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>