Bug 739505

retried with libvirt-0.9.4-11.el6.x86_64 + netcf-0.1.8-1.el6, after downgrade netcf and restart libvirtd, #virsh iface-dumpxml $vlan can get the correct result. So it's a netcf regression. 
There's already a Bug 736920 - [netcf][Regression] Command "virsh iface-dumpxml $BOND" will got an error with netcf-0.1.9-1, but not other version, which error is:
error: XML error: bond interface misses the bond element
both these two bugs belong to the same series. So please close it if it's a duplicate bug, thanks.

The following has been posted for review upstream (not showing up in the archives yet, so I can't just paste a link):

commit 2637ff98c0cd1ff6712068d990bc7f8e6ad13c30
Author: Laine Stump <laine>
Date:   Thu Sep 22 01:40:48 2011 -0400

    Fix missing vlan/bond/ethernet info in dumpxml --live
    
    This patch addresses:
    
      https://bugzilla.redhat.com/show_bug.cgi?id=736920
      https://bugzilla.redhat.com/show_bug.cgi?id=739505
    
    Commit 38f44cf (pushed between 0.1.8 and 0.1.9) put a check for an
    invalid ifindex into the functions that add type-specific status data
    to the xml document (e.g. the mac address of an ethernet, or tag of a
    vlan). This check was inadvertently done incorrectly ("ifindex !=
    RTNL_LINK_NOT_FOUND" rather than "ifindex == RTNL_LINK_NOT_FOUND").
    This patch corrects that error.
    
    Because these functions are a part of the code to gather info from the
    kernel via libnl (netlink) calls, there is unfortunately no simple way
    to incorporate a build-time test to prevent similar regressions in the
    future; we will instead have to rely on separate testing.

diff --git a/src/dutil_linux.c b/src/dutil_linux.c
index 75492ba..730be45 100644
--- a/src/dutil_linux.c
+++ b/src/dutil_linux.c
@@ -909,7 +909,7 @@ static void add_ethernet_info(struct netcf *ncf,
     struct rtnl_link *filter_link = NULL;
 
     /* if interface isn't currently available, nothing to add */
-    if (ifindex != RTNL_LINK_NOT_FOUND)
+    if (ifindex == RTNL_LINK_NOT_FOUND)
         return;
 
     filter_link = rtnl_link_alloc();
@@ -998,7 +998,7 @@ static void add_vlan_info(struct netcf *ncf,
     struct rtnl_link *filter_link = NULL;
 
     /* if interface isn't currently available, nothing to add */
-    if (ifindex != RTNL_LINK_NOT_FOUND)
+    if (ifindex == RTNL_LINK_NOT_FOUND)
         return;
 
     filter_link = rtnl_link_alloc();
@@ -1113,7 +1113,7 @@ static void add_bond_info(struct netcf *ncf,
         = { doc, root, NULL, ifindex, ncf };
 
     /* if interface isn't currently available, nothing to add */
-    if (ifindex != RTNL_LINK_NOT_FOUND)
+    if (ifindex == RTNL_LINK_NOT_FOUND)
         return;
 
     nl_cache_foreach(ncf->driver->link_cache, add_bond_info_cb, &cb_data);

A fix for this bug is available in a new netcf build for RHEL6:

  https://brewweb.devel.redhat.com/buildinfo?buildID=180923

tested with 
libvirt-0.9.4-13.el6.x86_64
netcf-0.1.9-2.el6.x86_64

Steps
[root@dhcp-93-106 ~]# virsh iface-define vlan.xml
Interface eth0.1 defined from vlan.xml

[root@dhcp-93-106 ~]# virsh iface-start eth0.1
Interface eth0.1 started

[root@dhcp-93-106 ~]# virsh iface-dumpxml eth0.1
<interface type='vlan' name='eth0.1'>
  <protocol family='ipv4'>
    <ip address='192.168.177.1' prefix='24'/>
  </protocol>
  <protocol family='ipv6'>
    <ip address='fe80::221:9bff:fe57:9f58' prefix='64'/>
  </protocol>
  <vlan tag='1'>
    <interface name='eth0'/>
  </vlan>
</interface>

[root@dhcp-93-106 ~]# virsh iface-destroy eth0.1
Interface eth0.1 destroyed

[root@dhcp-93-106 ~]# virsh iface-dumpxml eth0.1
<interface type='ethernet' name='eth0.1'>
</interface>


According to above steps , set bug status to VERIFIED

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2011-1631.html