More info on invocation from upstream commit messages:

For qemu:

    migration: dump vmstate info as a json file for static analysis
    
    This commit adds a new command, '-dump-vmstate', that takes a filename
    as an argument.  When executed, QEMU will dump the vmstate information
    for the machine type it's invoked with to the file, and quit.
    
    The JSON-format output can then be used to compare the vmstate info for
    different QEMU versions, specifically to test whether live migration
    would break due to changes in the vmstate data.
    
    A Python script that compares the output of such JSON dumps is included
    in the following commit.

and the script:

    vmstate-static-checker: script to validate vmstate changes
    
    This script compares the vmstate dumps in JSON format as output by QEMU
    with the -dump-vmstate option.
    
    It flags various errors, like version mismatch, sections going away,
    size mismatches, etc.
    
    This script is tolerant of a few changes that do not change the on-wire
    format, like embedding a few fields within substructs.
    
    The script takes -s/--src and -d/--dest parameters, to which filenames
    are given as arguments.
    
    Example:
    
    (in a qemu 2.0 tree):
    ./x86_64-softmmu/qemu-system-x86_64 -dump-vmstate qemu-2.0.json
    
    (in a qemu 2.2 tree:)
    ./x86_64-softmmu/qemu-system-x86_64 -dump-vmstate -M pc-i440fx-2.0 \
       qemu-2.2-m2.0.json
    
    ./scripts/vmstate-static-checker.py -s qemu-2.0.json -d qemu-2.2-m2.0.json
    
    The script also takes a --reverse parameter to switch the src and dest
    jsons.  This is just a shorthand for reversing the src and dest.
    
    The --help parameter shows usage information.

Fix included in qemu-kvm-0.12.1.2-2.435.el6

qemu-kvm-rhev-2.1.0-3.el7ev.preview.x86_64:

[root@dhcp-8-176 ~]# /usr/libexec/qemu-kvm -dump-vmstate pc.json -M rhel6.5.0
VNC server running on `::1:5900'


qemu-kvm-0.12.1.2-2.435.el6.x86_64:

[root@localhost 435]# /usr/libexec/qemu-kvm -dump-vmstate rhel6.5.0.json -M rhel6.5.0
VNC server running on `::1:5900'



[root@shu-fedora16 scripts]# ./vmstate-static-checker.py -s rhel6.5.0.json -d rhel6.5.0-RHEL7.json
Section "pcnet" does not exist in dest
Section "ES1370" does not exist in dest
Warning: checking incompatible machine types: "rhel6.6.0", "rhel6.5.0"
Section "ne2k_pci" does not exist in dest

Don't know whether "Warning: checking incompatible machine types: "rhel6.6.0", "rhel6.5.0"" is a problem.

(In reply to Shaolong Hu from comment #6)
> qemu-kvm-rhev-2.1.0-3.el7ev.preview.x86_64:
> 
> [root@dhcp-8-176 ~]# /usr/libexec/qemu-kvm -dump-vmstate pc.json -M rhel6.5.0
> VNC server running on `::1:5900'
> 
> 
> qemu-kvm-0.12.1.2-2.435.el6.x86_64:
> 
> [root@localhost 435]# /usr/libexec/qemu-kvm -dump-vmstate rhel6.5.0.json -M
> rhel6.5.0
> VNC server running on `::1:5900'
> 
> 
> 
> [root@shu-fedora16 scripts]# ./vmstate-static-checker.py -s rhel6.5.0.json
> -d rhel6.5.0-RHEL7.json
> Section "pcnet" does not exist in dest
> Section "ES1370" does not exist in dest
> Warning: checking incompatible machine types: "rhel6.6.0", "rhel6.5.0"
> Section "ne2k_pci" does not exist in dest
> 
> Don't know whether "Warning: checking incompatible machine types:
> "rhel6.6.0", "rhel6.5.0"" is a problem.

The warning is by my mistake, it works correctly:

[root@shu-fedora16 scripts]# ./vmstate-static-checker.py -s rhel6.5.0.json -d rhel6.5.0-RHEL7.json
Section "pcnet" does not exist in dest
Section "ES1370" does not exist in dest
Section "ne2k_pci" does not exist in dest

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-2014-1490.html