xen/vcpu: Simplify xen_vcpu related code

Largely mechanical changes to aid unification of xen_vcpu_restore()
logic for PV, PVH and PVHVM.

xen_vcpu_setup(): the only change in logic is that clamp_max_cpus()
is now handled inside the "if (!xen_have_vcpu_info_placement)" block.

xen_vcpu_restore(): code movement from enlighten_pv.c to enlighten.c.

xen_vcpu_info_reset(): pulls together all the code where xen_vcpu
is set to default.

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
diff --git a/arch/x86/xen/smp_hvm.c b/arch/x86/xen/smp_hvm.c
index f18561b..9e0fb9a 100644
--- a/arch/x86/xen/smp_hvm.c
+++ b/arch/x86/xen/smp_hvm.c
@@ -12,7 +12,8 @@
 	native_smp_prepare_boot_cpu();
 
 	/*
-	 * Setup vcpu_info for boot CPU.
+	 * Setup vcpu_info for boot CPU. Secondary CPUs get their vcpu_info
+	 * in xen_cpu_up_prepare_hvm().
 	 */
 	xen_vcpu_setup(0);