x86/shadow: adjust monitor table prealloc amount

While 670d6b908f ('x86 shadow: Move the shadow linear mapping for
n-on-3-on-4 shadows so') bumped the amount by one too little for the
32-on-64 case (which luckily was dead code, and hence a bump wasn't
necessary in the first place), 0b841314da ('x86/shadow:
sh_{make,destroy}_monitor_table() are "even more" HVM-only'), dropping
the dead code, then didn't adjust the amount back. Yet even the original
amount was too high in certain cases. Switch to pre-allocating just as
much as is going to be needed.

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

xen/arch/x86/mm/shadow/hvm.c

@@ -747,7 +747,7 @@ mfn_t sh_make_monitor_table(const struct vcpu *v, unsigned int shadow_levels)
     ASSERT(!pagetable_get_pfn(v->arch.hvm.monitor_table));
 
     /* Guarantee we can get the memory we need */
-    if ( !shadow_prealloc(d, SH_type_monitor_table, CONFIG_PAGING_LEVELS) )
+    if ( !shadow_prealloc(d, SH_type_monitor_table, shadow_levels < 4 ? 3 : 1) )
         return INVALID_MFN;
 
     m4mfn = shadow_alloc(d, SH_type_monitor_table, 0);