i386: clarify 'hv-passthrough' behavior

Clarify the fact that 'hv-passthrough' only enables features which are
already known to QEMU and that it overrides all other 'hv-*' settings.

Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210608120817.1325125-3-vkuznets@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

docs/hyperv.txt

@@ -209,8 +209,11 @@ In some cases (e.g. during development) it may make sense to use QEMU in
 'pass-through' mode and give Windows guests all enlightenments currently
 supported by KVM. This pass-through mode is enabled by "hv-passthrough" CPU
 flag.
-Note: enabling this flag effectively prevents migration as supported features
-may differ between target and destination.
+Note: "hv-passthrough" flag only enables enlightenments which are known to QEMU
+(have corresponding "hv-*" flag) and copies "hv-spinlocks="/"hv-vendor-id="
+values from KVM to QEMU. "hv-passthrough" overrides all other "hv-*" settings on
+the command line. Also, enabling this flag effectively prevents migration as the
+list of enabled enlightenments may differ between target and destination hosts.
 
 
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