x86/boot: Improve paging mode diagnostics in create_dom0()

I was presented with this:

  (XEN) NX (Execute Disable) protection active
  (XEN) d0 has maximum 416 PIRQs
  (XEN)
  (XEN) ****************************************
  (XEN) Panic on CPU 0:
  (XEN) Error creating d0: -95
  (XEN) ****************************************

which is less than helpful.  It turns out to be the -EOPNOTSUPP from
shadow_domain_init().

The real bug here is create_dom0() unconditionally assuming the presence of
SHADOW_PAGING.  Rework it to panic() rather than choosing a dom0_cfg which is
guaranteed to fail.  This results in:

  (XEN) NX (Execute Disable) protection active
  (XEN)
  (XEN) ****************************************
  (XEN) Panic on CPU 0:
  (XEN) Neither HAP nor Shadow available for PVH domain
  (XEN) ****************************************

which is rather more helpful.

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

xen/arch/x86/setup.c

@@ -1033,9 +1033,12 @@ static struct domain *__init create_dom0(struct boot_info *bi)
 
     if ( opt_dom0_pvh )
     {
-        dom0_cfg.flags |= (XEN_DOMCTL_CDF_hvm |
-                           ((hvm_hap_supported() && !opt_dom0_shadow) ?
-                            XEN_DOMCTL_CDF_hap : 0));
+        dom0_cfg.flags |= XEN_DOMCTL_CDF_hvm;
+
+        if ( hvm_hap_supported() && !opt_dom0_shadow )
+            dom0_cfg.flags |= XEN_DOMCTL_CDF_hap;
+        else if ( !IS_ENABLED(CONFIG_SHADOW_PAGING) )
+            panic("Neither HAP nor Shadow available for PVH domain\n");
 
         dom0_cfg.arch.emulation_flags |=
             XEN_X86_EMU_LAPIC | XEN_X86_EMU_IOAPIC | XEN_X86_EMU_VPCI;