conf: Introduce optional startupPolicy attribute for cdrom and floppy

This attribute says what to do with cdrom (or floppy) if
the source is missing. It accepts:
- mandatory - fail if missing for any reason (the default)
- requisite - fail if missing on boot up, drop if missing on
              migrate/restore/revert
- optional  - drop if missing at any start attempt.

However, this patch introduces only XML part of this new
functionality.

tests/qemuxml2xmloutdata/qemuxml2xmlout-disk-cdrom-empty.xml

@@ -0,0 +1,31 @@
+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <memory>219100</memory>
+  <currentMemory>219100</currentMemory>
+  <vcpu>1</vcpu>
+  <os>
+    <type arch='i686' machine='pc'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu</emulator>
+    <disk type='block' device='disk'>
+      <source dev='/dev/HostVG/QEMUGuest1'/>
+      <target dev='hda' bus='ide'/>
+      <address type='drive' controller='0' bus='0' unit='0'/>
+    </disk>
+    <disk type='file' device='cdrom'>
+      <target dev='hdc' bus='ide'/>
+      <source startupPolicy='optional'/>
+      <readonly/>
+      <address type='drive' controller='0' bus='1' unit='0'/>
+    </disk>
+    <controller type='ide' index='0'/>
+    <memballoon model='virtio'/>
+  </devices>
+</domain>