conf: don't ignore <target dev='blah'/> for macvtap interfaces

The parser had been clearing out *all* suggested device names for
type='direct' (aka macvtap) interfaces. All of the code implementing
macvtap allows for a user-specified device name, so we should allow
it. In the case that an interface name starts with "macvtap" or
"macvlan" though, we do still clear it out, just as we do with "vnet"
(which is the prefix used for automatically generated tap device
names), since those are the prefixes for the names we autogenerate for
macvtap and macvlan devices.

Resolves: https://bugzilla.redhat.com/1335798

docs/formatdomain.html.in

@@ -5210,9 +5210,9 @@ qemu-kvm -net nic,model=? /dev/null
       If no target is specified, certain hypervisors will
       automatically generate a name for the created tun device. This
       name can be manually specified, however the name <i>should not
-      start with either 'vnet' or 'vif'</i>, which are prefixes
-      reserved by libvirt and certain hypervisors. Manually specified
-      targets using these prefixes may be ignored.
+      start with either 'vnet', 'vif', 'macvtap', or 'macvlan'</i>,
+      which are prefixes reserved by libvirt and certain hypervisors.
+      Manually specified targets using these prefixes may be ignored.
     </p>
 
     <p>

src/conf/domain_conf.c

@@ -55,6 +55,7 @@
 #include "virtpm.h"
 #include "virstring.h"
 #include "virnetdev.h"
+#include "virnetdevmacvlan.h"
 #include "virhostdev.h"
 #include "virmdev.h"
 
@@ -10029,8 +10030,12 @@ virDomainNetDefParseXML(virDomainXMLOptionPtr xmlopt,
         def->data.direct.linkdev = dev;
         dev = NULL;
 
-        if (flags & VIR_DOMAIN_DEF_PARSE_INACTIVE)
+        if (ifname &&
+            flags & VIR_DOMAIN_DEF_PARSE_INACTIVE &&
+            (STRPREFIX(ifname, VIR_NET_GENERATED_MACVTAP_PREFIX) ||
+             STRPREFIX(ifname, VIR_NET_GENERATED_MACVTAP_PREFIX))) {
             VIR_FREE(ifname);
+        }
 
         break;