docs: document that vfio is default for hostdev networks too

When the default was changed from kvm to vfio, the documentation for
hostdev and interface was changed, but the documentation in <network>
was forgotten.

Also document when the default was changed from "always kvm" to "vfio
if available, else kvm" (1.0.5).

docs/formatdomain.html.in

@@ -2788,12 +2788,16 @@
         device assignment. Use the <code>name</code> attribute to
         select either "vfio" (for the new VFIO device assignment
         backend, which is compatible with UEFI SecureBoot) or "kvm"
-        (for the legacy device assignment handled directly by the KVM
+        (the legacy device assignment handled directly by the KVM
         kernel module)<span class="since">Since 1.0.5 (QEMU and KVM
-        only, requires kernel 3.6 or newer)</span>. The default, when
-        the driver name is not explicitly specified, is to check whether
-        VFIO is available and use it if it's the case. If VFIO is not
-        available, the legacy "kvm" assignment is attempted.
+        only, requires kernel 3.6 or newer)</span>. When specified,
+        device assignment will fail if the requested method of device
+        assignment isn't available on the host. When not specified,
+        the default is "vfio" on systems where the VFIO driver is
+        available and loaded, and "kvm" on older systems, or those
+        where the VFIO driver hasn't been
+        loaded <span class="since">Since 1.1.3</span> (prior to that
+        the default was always "kvm").
       </dd>
       <dt><code>readonly</code></dt>
       <dd>Indicates that the device is readonly, only supported by SCSI host

docs/formatnetwork.html.in

@@ -282,17 +282,22 @@
             definition. <span class="since"> Since 0.10.0</span>
 
             <p>
-              To use VFIO device assignment rather than
-              traditional/legacy KVM device assignment (VFIO is a new
-              method of device assignment that is compatible with UEFI
-              Secure Boot), a &lt;forward type='hostdev'&gt; interface
-              can have an optional <code>driver</code> sub-element
-              with a <code>name</code> attribute set to "vfio". To use
-              legacy KVM device assignment you can
-              set <code>name</code> to "kvm" (or simply omit the
-              &lt;driver&gt; element, since "kvm" is currently the
-              default).
-              <span class="since">Since 1.0.5 (QEMU and KVM only, requires kernel 3.6 or newer)</span>
+              To force use of a particular type of device assignment,
+              a &lt;forward type='hostdev'&gt; interface can have an
+              optional <code>driver</code> sub-element with
+              a <code>name</code> attribute set to either "vfio" (VFIO
+              is a new method of device assignment that is compatible
+              with UEFI Secure Boot) or "kvm" (the legacy device
+              assignment handled directly by the KVM kernel module)
+              <span class="since">Since 1.0.5 (QEMU and KVM only,
+              requires kernel 3.6 or newer)</span>. When specified,
+              device assignment will fail if the requested method of
+              device assignment isn't available on the host. When not
+              specified, the default is "vfio" on systems where the
+              VFIO driver is available and loaded, and "kvm" on older
+              systems, or those where the VFIO driver hasn't been
+              loaded <span class="since">Since 1.1.3</span> (prior to
+              that the default was always "kvm").
             </p>
 
             <p>Note that this "intelligent passthrough" of network