cli: Don't overwrite install bootorder with manual --boot (bz 1438946)

--boot should be for post-install bootorder only

https://bugzilla.redhat.com/show_bug.cgi?id=1438946

tests/cli-test-xml/compare/virt-install-singleton-config-2.xml

@@ -60,9 +60,6 @@
   <os>
     <type arch="x86_64">hvm</type>
     <loader>/foo/bar</loader>
-    <boot dev="cdrom"/>
-    <boot dev="fd"/>
-    <boot dev="hd"/>
     <boot dev="network"/>
     <smbios mode="sysinfo"/>
     <bootmenu enable="no"/>

virtinst/installer.py

@@ -113,13 +113,13 @@ class Installer(object):
         if isinstall and not self.has_install_phase():
             return
 
-        bootorder = self._build_boot_order(isinstall, guest)
-
-        if not guest.os.bootorder:
+        bootorder = guest.os.bootorder
+        if isinstall or not bootorder:
             # Per device <boot order> is not compatible with os/boot.
             if not any(d.boot.order for d in guest.get_all_devices()):
-                guest.os.bootorder = bootorder
+                bootorder = self._build_boot_order(isinstall, guest)
 
+        guest.os.bootorder = bootorder
         if not isinstall:
             return