qemu: Support setting the 'removable' flag for USB disks

Add an attribute named 'removable' to the 'target' element of disks,
which controls the removable flag. For instance, on a Linux guest it
controls the value of /sys/block/$dev/removable. This option is only
valid for USB disks (i.e. bus='usb'), and its default value is 'off',
which is the same behaviour as before.

To achieve this, 'removable=on' (or 'off') is appended to the '-device
usb-storage' parameter sent to qemu when adding a USB disk via
'-disk'. A capability flag QEMU_CAPS_USB_STORAGE_REMOVABLE was added
to keep track if this option is supported by the qemu version used.

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=922495
Signed-off-by: Peter Krempa <pkrempa@redhat.com>

docs/formatdomain.html.in

@@ -1792,9 +1792,13 @@
         removable disks (i.e. CDROM or Floppy disk), the value can be either
         "open" or "closed", defaults to "closed". NB, the value of
         <code>tray</code> could be updated while the domain is running.
-        <span class="since">Since 0.0.3; <code>bus</code> attribute since 0.4.3;
+        The optional attribute <code>removable</code> sets the
+        removable flag for USB disks, and its value can be either "on"
+        or "off", defaulting to "off". <span class="since">Since
+        0.0.3; <code>bus</code> attribute since 0.4.3;
         <code>tray</code> attribute since 0.9.11; "usb" attribute value since
-        after 0.4.4; "sata" attribute value since 0.9.7</span>
+        after 0.4.4; "sata" attribute value since 0.9.7; "removable" attribute
+        value since 1.1.3</span>
       </dd>
       <dt><code>iotune</code></dt>
       <dd>The optional <code>iotune</code> element provides the

docs/schemas/domaincommon.rng

@@ -1287,6 +1287,14 @@
           </choice>
         </attribute>
       </optional>
+      <optional>
+        <attribute name="removable">
+          <choice>
+            <value>on</value>
+            <value>off</value>
+          </choice>
+        </attribute>
+      </optional>
     </element>
   </define>
   <define name="geometry">