xen: fix kdump kernel crash on Xen3.2

xen: fix kdump kernel crash on Xen3.2

The kernel is supposed to create some "Crash note" resources (children
of the "Hypervisor code and data" resource in /proc/iomem).  However,
when running on Xen 3.2, xen_machine_kexec_setup_resources()
encounters an error and returns prior to doing this.

The error occurs when it calls the "kexec_get_range" hypercall to
determine the location of the "vmcoreinfo".  This was only implemented
in Xen 3.3.

This patch makes the kernel handle this error gracefully by simply not
creating the sysfs file "hypervisor/vmcoreinfo" if the hypervisor is
unable to provide the info - rather than bailing out of
xen_machine_kexec_setup_resources() early.

Signed-off-by: Alex Zeffertt <alex.zeffertt@eu.citrix.com>