x86/dom0: enable pf-fixup by default for PVH dom0

A non-trivial amount of issues related to PVH dom0 are fixed by enabling
the `pf-fixup` command line option, and no issues have been reported as a
result of its usage.  Enable the option by default to make PVH dom0 more
robust in its default configuration.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

docs/misc/xen-command-line.pandoc

@@ -849,7 +849,7 @@ Controls for how dom0 is constructed on x86 systems.
     If using this option is necessary to fix an issue, please report a bug.
 
 *   The `pf-fixup` boolean is only applicable when using a PVH dom0 and
-    defaults to false.
+    defaults to true.
 
     When running dom0 in PVH mode the dom0 kernel has no way to map MMIO
     regions into its physical memory map, such mode relies on Xen dom0 builder
@@ -857,10 +857,14 @@ Controls for how dom0 is constructed on x86 systems.
     access.  However Xen doesn't have a complete picture of the host memory
     map, due to not being able to process ACPI dynamic tables.
 
-    The `pf-fixup` option allows Xen to attempt to add missing MMIO regions
+    The `pf-fixup` option causes Xen to attempt to add missing MMIO regions
     to the dom0 physical memory map in response to page-faults generated by
     dom0 trying to access unpopulated entries in the memory map.
 
+    Users having to disable the feature to boot in PVH dom0 mode should report
+    it to the xen-devel mailing list, together with Xen and dom0 output with the
+    option enabled.
+
 Enables features on dom0 on Arm systems.
 
 *   The `sve` integer parameter enables Arm SVE usage for Dom0 and sets the