vm: extend shared property to support locking

The drives "shared" property has been extended (from True/False) to
support sanlock locking.

The new values are:

 - "none":      no infomation (locking disabled)
 - "exclusive": the disk is used by the VM in exclusive mode
 - "shared":    the disk might be used by multiple VMs

For backward compatibility the old values are mapped as follows:

 - True: "shared"
 - False: "none" (or "exclusive" if use_volume_leases is True)

Change-Id: I9429ead45caac1178957a33393642817db59508f
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17714
Reviewed-by: Ayal Baron <abaron@redhat.com>