Description of problem:

Luci doesn't save configuration correctly for Virtual Machine service once a user decides to clear options that were previously configured.

When trying to save an empty value for an option that used to be non-empty before, instead of removing such option from cluster.conf it is saved with the previous non-empty value. That makes it impossible for a user to change the configuration without touching cluster.conf by hand. This bug affects all of the service-specific options (except "Migration Type").

These options are affected:
 - Migration Mapping
 - Status Program
 - Path to xmlfile Used to Create the VM
 - VM Configuration File Path
 - Path to the VM Snapshot Directory
 - Hypervisor URI
 - Migration URI
 - Independent Subtree / Non-Critical Resource


Version-Release number of selected component (if applicable):

luci-0.23.0-20.el6


How reproducible:

Always


Steps to Reproduce:
1. Create a simple cluster using luci web GUI
2. Create a new Service group of any name for this cluster created at step 1, submit changes
3. Add a new Virtual Machine resource to the service group created above, submit changes
4. Check cluster.conf:

[root@z4 cluster]# cat cluster.conf |grep "config_version\|vm"
	<cluster config_version="3" name="zcluster">
	<vm autostart="0" migrate="live" name="virtual" recovery="relocate"/>

5. Open the service configuration again and fill in any or all of the following option fields (values I used here are just an example):
 - Migration Mapping
 - Status Program
 - Path to xmlfile Used to Create the VM
 - VM Configuration File Path
 - Path to the VM Snapshot Directory
 - Hypervisor URI
 - Migration URI
 - Independent Subtree / Non-Critical Resource
6. Submit changes
7. Check cluster.conf again to see as they're correctly saved:

[root@z4 cluster]# cat cluster.conf |grep "config_version\|vm"
	<cluster config_version="4" name="zcluster">
	<vm __independent_subtree="2" autostart="0" hypervisor_uri="qemu:///system" migrate="live" migration_mapping="node1:example1,node2:example2" migration_uri="qemu+ssh://target_host/system" name="virtual" path="/etc/libvirt/qemu" recovery="relocate" snapshot="/mnt/gfs" status_program="/usr/local/bin/status.sh" xmlfile="/etc/libvirt/qemu/guest.xml"/>

8. Open the service configuration for the last time and clear any or all of the fields filled in step 5. Submit changes.
9. Check if the options have been cleared in cluster.conf.
  
Actual results:

Options are not cleared, they're saved with old values instead. No error message is shown:

[root@z4 cluster]# cat cluster.conf |grep "config_version\|vm"
	<cluster config_version="5" name="zcluster">
	<vm __independent_subtree="2" autostart="0" hypervisor_uri="qemu:///system" migrate="live" migration_mapping="node1:example1,node2:example2" migration_uri="qemu+ssh://target_host/system" name="virtual" path="/etc/libvirt/qemu" recovery="relocate" snapshot="/mnt/gfs" status_program="/usr/local/bin/status.sh" xmlfile="/etc/libvirt/qemu/guest.xml"/>


Expected results:

Options are cleared:

[root@z4 cluster]# cat cluster.conf |grep "config_version\|vm"
	<cluster config_version="5" name="zcluster">
	<vm autostart="0" migrate="live" name="virtual" recovery="relocate"/>


Additional info:

Tested in Firefox 6, Google Chrome 15.0.854 and Epiphany 3.0.3