xen: fix backport of previous kexec patch

Fixes the backport of 0b34a166 upstream

Commit 0b34a166

 "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>