storage: Process storage pool capabilities

storage: Process storage pool capabilities

https://bugzilla.redhat.com/show_bug.cgi?id=1581670

During storage driver backend initialization, let's save
which backends are available in the storage pool capabilities.

In order to format those, we need add a connectGetCapabilities
processor to the storageHypervisorDriver. This allows a storage
connection, such as "storage:///system" to find the API and
format the results, such as:

  virsh -c storage:///system capabilities

  <capabilities>

    <pool>
      <enum name='type'>
        <value>dir</value>
        <value>fs</value>
        <value>netfs</value>
        <value>logical</value>
        <value>iscsi</value>
        <value>iscsi-direct</value>
        <value>scsi</value>
        <value>mpath</value>
        <value>disk</value>
        <value>rbd</value>
        <value>sheepdog</value>
        <value>gluster</value>
        <value>zfs</value>
      </enum>
    </pool>

  </capabilities>

Signed-off-by: John Ferlan <jferlan@redhat.com>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>