Description of problem:
Can create a posixfs-based datacenter. However, posixfs doesn't seem to be listed under <storage_types/> in capabilities.

Version-Release number of selected component (if applicable):
3.1

How reproducible:
Always

Steps to Reproduce:
1. Perform a GET request on /api/capabilities
2. Check <storage_types/>
3.
  
Actual results:

<storage_types>
  <storage_type>iscsi</storage_type>
  <storage_type>fcp</storage_type>
  <storage_type>nfs</storage_type>
  <storage_type>localfs</storage_type>
</storage_types>

Expected results:

<storage_types>
  <storage_type>iscsi</storage_type>
  <storage_type>fcp</storage_type>
  <storage_type>nfs</storage_type>
  <storage_type>localfs</storage_type>
  <storage_type>posixfs</storage_type>
</storage_types>


Additional info: