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

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>
OraBug: 25497392

(cherry picked from commit 5b00b504b13b2f0d1aa73d59cf8984726f19100f)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>