xen: rename xen_pmu_init() in sys-hypervisor.c

There are two functions with name xen_pmu_init() in the kernel. Rename
the one in drivers/xen/sys-hypervisor.c to avoid shadowing the one in
arch/x86/xen/pmu.c

To avoid the same problem in future rename some more functions.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>