qemu: Prefer VFIO for PCI device passthrough

Prefer using VFIO (if available) to the legacy KVM device passthrough.

With this patch a PCI passthrough device without the driver configured
will be started with VFIO if it's available on the host. If not legacy
KVM passthrough is checked and error is reported if it's not available.

docs/formatdomain.html.in

@@ -2757,11 +2757,10 @@
         backend, which is compatible with UEFI SecureBoot) or "kvm"
         (for 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>. Currently, "kvm"
-        is the default used by libvirt when not explicitly provided,
-        but since the two are functionally equivalent, this default
-        could be changed in the future with no impact to domains that
-        don't specify anything.
+        only, requires kernel 3.6 or newer)</span>. The default, when
+        the driver name is not explicitly specified, is to check wether
+        VFIO is available and use it if it's the case. If VFIO is not
+        available, the legacy "kvm" assignment is attempted.
       </dd>
       <dt><code>readonly</code></dt>
       <dd>Indicates that the device is readonly, only supported by SCSI host