conf: Add rom.enabled attribute for PCI devices

The attribute can be used to disable ROM loading completely
for a device.

This might be needed because, even when the guest is configured
such that the PCI ROM will not be loaded in the PCI BAR, some
hypervisors (eg. QEMU) might still make it available to the
guest in a form (eg. fw_cfg) that some firmwares (eg. SeaBIOS)
will consume, thus not achieving the desired result.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>

docs/formatdomain.html.in

@@ -4476,6 +4476,12 @@
         virtual function of an sr-iov capable ethernet device (which
         has no boot ROMs for the VFs).
         <span class="since">Since 0.9.10 (QEMU and KVM only)</span>.
+        The optional <code>enabled</code> attribute can be set to
+        <code>no</code> to disable PCI ROM loading completely for the device;
+        if PCI ROM loading is disabled through this attribute, attempts to
+        tweak the loading process further using the <code>bar</code> or
+        <code>file</code> attributes will be rejected.
+        <span class="since">Since 4.3.0 (QEMU and KVM only)</span>.
       </dd>
       <dt><code>address</code></dt>
       <dd>The <code>address</code> element for USB devices has a

docs/schemas/domaincommon.rng

@@ -5108,6 +5108,11 @@
 
   <define name="rom">
     <element name="rom">
+      <optional>
+        <attribute name="enabled">
+          <ref name="virYesNo"/>
+        </attribute>
+      </optional>
       <optional>
         <attribute name="bar">
           <ref name="virOnOff"/>