storage: ZFS support

Implement ZFS storage backend driver. Currently supported
only on FreeBSD because of ZFS limitations on Linux.

Features supported:

 - pool-start, pool-stop
 - pool-info
 - vol-list
 - vol-create / vol-delete

Pool definition looks like that:

 <pool type='zfs'>
  <name>myzfspool</name>
  <source>
    <name>actualpoolname</name>
  </source>
 </pool>

The 'actualpoolname' value is a name of the pool on the system,
such as shown by 'zpool list' command. Target makes no sense
here because volumes path is always /dev/zvol/$poolname/$volname.

User has to create a pool on his own, this driver doesn't
support pool creation currently.

A volume could be used with Qemu by adding an entry like this:

    <disk type='volume' device='disk'>
      <driver name='qemu' type='raw'/>
      <source pool='myzfspool' volume='vol5'/>
      <target dev='hdc' bus='ide'/>
    </disk>

tools/virsh-pool.c

@@ -1063,6 +1063,9 @@ cmdPoolList(vshControl *ctl, const vshCmd *cmd ATTRIBUTE_UNUSED)
             case VIR_STORAGE_POOL_GLUSTER:
                 flags |= VIR_CONNECT_LIST_STORAGE_POOLS_GLUSTER;
                 break;
+            case VIR_STORAGE_POOL_ZFS:
+                flags |= VIR_CONNECT_LIST_STORAGE_POOLS_ZFS;
+                break;
             case VIR_STORAGE_POOL_LAST:
                 break;
             }