Initial POST body XML support

[mfojtik]

Hi,

this patch add possibility to create a new instance using POST body XML.

How it works:

instance.xml:
<instance>
  <name>testinst10</name>
  <realm id="us"/>
  <hardware_profile id="m1-small">
    <memory>512</memory>
    <cpu>2</cpu>
    <storage>10</storage>
    <architecture>i386</architecture>
  </hardware_profile>
</instance>

Then:

curl -v -X POST -d "`cat instance.xml`" -H 'Accept: application/xml' 
http://mockuser:mockpassword@localhost:3001/api/instances

You should get a create instance as reponse.
Let me know what do you think. If we gonna support this way, it should be easy 
to do a BlockDeviceMappings or handling
firewall configurations.

  -- Michal