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description of problem:
libvirt should improve the xml format when define a vm have a empty <cpu/>

Version-Release number of selected component (if applicable):
libvirt-1.2.8-16.el7_1.1.x86_64
qemu-kvm-rhev-2.1.2-23.el7_1.1.x86_64

How reproducible:
100%

Steps to Reproduce:
 
1.edit a vm like this:
# virsh edit test3
...
  <cpu>
  </cpu>
...

2.check the xml
# virsh dumpxml test3
...
  <cpu>
  </cpu>
...

Actual results:
libvirt still format a empty <cpu/> and do not show any useful information in it.

Expected results:
if <cpu/> do not contain any special means in it, no need format it in xml, it will waste time to parse it.
if <cpu/> contain some special things, we should show them clearly in the xml.

Additional info:

This has been fixed upstream with the following commit.

commit e4d7ddfdd2220046b8ca9f5e2af3a5d45e962e44
Author: Andrea Bolognani <abologna>
Date:   Fri Apr 10 15:09:59 2015 +0200

    conf: Don't output <cpu> tag if it contains no information.
    
    The tag is already marked as optional in the schema, so no changes
    are needed there.


v1.2.14-107-ge4d7ddf

Verify this bug with libvirt-1.2.16-1.el7.x86_64:

1 . add a empty cpu element in guest xml:

# virsh edit test4
...
  <cpu>
  </cpu>
...

2. recheck the xml after edit:

# virsh dumpxml test4 |grep "<cpu>"
# echo $?
1

retest with libvirt-1.2.17-1.el7.x86_64 and get the same result.

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.

https://rhn.redhat.com/errata/RHBA-2015-2202.html