xen: fix backport of previous kexec patch

xen: fix backport of previous kexec patch

Orabug: 22623832

Fixes the backport of 0b34a166f291d255755be46e43ed5497cdd194f2 upstream

Commit 0b34a166f291d255755be46e43ed5497cdd194f2 "x86/xen: Support
kexec/kdump in HVM guests by doing a soft reset" has been added to the
4.2-stable tree" needed to correct the CONFIG variable, as
CONFIG_KEXEC_CORE only showed up in 4.3.

Reported-by: David Vrabel <david.vrabel@citrix.com>
Reported-by: Luis Henriques <luis.henriques@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit e4338aeff6aab3640c2925844f4e5e53746cd3d9)
Signed-off-by: Dan Duval <dan.duval@oracle.com>