x86/pv: provide more helpful error when CONFIG_PV32 is absent

Currently, when booting a 32bit dom0 kernel, the message isn't very
helpful:

  (XEN)  Xen  kernel: 64-bit, lsb
  (XEN)  Dom0 kernel: 32-bit, PAE, lsb, paddr 0x100000 -> 0x112000
  (XEN) Mismatch between Xen and DOM0 kernel
  (XEN)
  (XEN) ****************************************
  (XEN) Panic on CPU 0:
  (XEN) Could not construct domain 0
  (XEN) ****************************************

With this adjustment, it now looks like this:

  (XEN)  Xen  kernel: 64-bit, lsb
  (XEN) Found 32-bit PV kernel, but CONFIG_PV32 missing
  (XEN)
  (XEN) ****************************************
  (XEN) Panic on CPU 0:
  (XEN) Could not construct domain 0
  (XEN) ****************************************

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jane Malalane <jane.malalane@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/pv/dom0_build.c

@@ -362,9 +362,9 @@ int __init dom0_construct_pv(struct domain *d,
     compatible = false;
     machine = elf_uval(&elf, elf.ehdr, e_machine);
 
-#ifdef CONFIG_PV32
     if ( elf_32bit(&elf) )
     {
+#ifdef CONFIG_PV32
         if ( parms.pae == XEN_PAE_BIMODAL )
             parms.pae = XEN_PAE_EXTCR3;
         if ( parms.pae && machine == EM_386 )
@@ -377,8 +377,11 @@ int __init dom0_construct_pv(struct domain *d,
 
             compatible = true;
         }
-    }
+#else
+        printk("Found 32-bit PV kernel, but CONFIG_PV32 missing\n");
+        return -EOPNOTSUPP;
 #endif
+    }
 
     compat = is_pv_32bit_domain(d);