-machine vmport=auto: Fix handling of VMWare ioport emulation for xen

c/s 9b23cfb76b

or

c/s b154537ad0

moved the testing of xen_enabled() from pc_init1() to
pc_machine_initfn().

xen_enabled() does not return the correct value in
pc_machine_initfn().

Changed vmport from a bool to an enum.  Added the value "auto" to do
the old way.  Move check of xen_enabled() back to pc_init1().

Acked-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Don Slutz <dslutz@verizon.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qapi/common.json

@@ -87,3 +87,18 @@
 ##
 { 'command': 'query-commands', 'returns': ['CommandInfo'] }
 
+##
+# @OnOffAuto
+#
+# An enumeration of three options: on, off, and auto
+#
+# @auto: QEMU selects the value between on and off
+#
+# @on: Enabled
+#
+# @off: Disabled
+#
+# Since: 2.2
+##
+{ 'enum': 'OnOffAuto',
+  'data': [ 'auto', 'on', 'off' ] }