Description of problem:
libvirt doesn't handle vmdk images with the following subformats:
    - monolithicFlat
    - twoGbMaxExtentSparse
    - twoGbMaxExtentFlat

Version-Release number of selected component (if applicable):
libvirt-1.1.1-12.el7.x86_64
qemu-kvm-1.5.3-19.el7.x86_64

How reproducible:
100%

Steps to Reproduce:

1. Use the qemu-img create the vmdk images:

# qemu-img create -f vmdk test1.vmdk -o subformat=monolithicFlat 2G
Formatting 'test1.vmdk', fmt=vmdk size=2147483648 compat6=off subformat='monolithicFlat' zeroed_grain=off 

# qemu-img create -f vmdk test3.vmdk -o subformat=twoGbMaxExtentSparse 2G
Formatting 'test3.vmdk', fmt=vmdk size=2147483648 compat6=off subformat='twoGbMaxExtentSparse' zeroed_grain=off 

# qemu-img create -f vmdk test4.vmdk -o subformat=twoGbMaxExtentFlat 2G
Formatting 'test4.vmdk', fmt=vmdk size=2147483648 compat6=off subformat='twoGbMaxExtentFlat' zeroed_grain=off 

2. Check the format in qemu-img info, the files' format are vmdk, and the size are 2.0G

3. Check these disks' info in libvirt. The format are raw, and the capacity are not 2G.

# virsh vol-dumpxml /var/lib/libvirt/images/test1.vmdk 
<volume>
  <name>test1.vmdk</name>
  <key>/var/lib/libvirt/images/test1.vmdk</key>
  <source>
  </source>
  <capacity unit='bytes'>315</capacity>
  <allocation unit='bytes'>4096</allocation>
  <target>
    <path>/var/lib/libvirt/images/test1.vmdk</path>
    <format type='raw'/>
......

# virsh vol-dumpxml /var/lib/libvirt/images/test3.vmdk 
<volume>
  <name>test3.vmdk</name>
  <key>/var/lib/libvirt/images/test3.vmdk</key>
  <source>
  </source>
  <capacity unit='bytes'>321</capacity>
  <allocation unit='bytes'>4096</allocation>
  <target>
    <path>/var/lib/libvirt/images/test3.vmdk</path>
    <format type='raw'/>
......

# virsh vol-dumpxml /var/lib/libvirt/images/test4.vmdk 
<volume>
  <name>test4.vmdk</name>
  <key>/var/lib/libvirt/images/test4.vmdk</key>
  <source>
  </source>
  <capacity unit='bytes'>319</capacity>
  <allocation unit='bytes'>4096</allocation>
  <target>
    <path>/var/lib/libvirt/images/test4.vmdk</path>
    <format type='raw'/>
......

Current results:
In step3 the format are raw, and the capacity are not 2G.

Expect results:
In step3 the format are vmdk, and the capacity are 2G.