package/xen: add upstream security fix for XSA-327

Fixes the following security issue:

CVE-2020-15564: Missing alignment check in VCPUOP_register_vcpu_info

For further details, see the advisory:

https://xenbits.xenproject.org/xsa/advisory-327.html



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>