Description of problem:
rbd pool should support vol-create a encrypted volume

Version-Release number of selected component (if applicable):
libvirt-2.0.0-8.el7.x86_64

How reproducible:
100%

This is from 
https://bugzilla.redhat.com/show_bug.cgi?id=1301021#c9


Steps to Reproduce:
# virsh pool-dumpxml rbd
<pool type='rbd'>
  <name>rbd</name>
  <uuid>ab987a2a-e5c1-4b67-ad2f-ea2801541766</uuid>
  <capacity unit='bytes'>152820314112</capacity>
  <allocation unit='bytes'>260358</allocation>
  <available unit='bytes'>61114695680</available>
  <source>
    <host name='10.x.x.x' port='6789'/>
    <host name='10.x.x.x' port='6789'/>
    <name>yisun-pool</name>
  </source>
</pool>

And I edit a vol xml:
# cat rbd_vol1.xml 
<volume type='network'>
<name>luks_vol1.img</name>
<source>
</source>
<capacity unit='bytes'>6368709120</capacity>
<allocation unit='bytes'>6368709120</allocation>
<target>
<format type='raw'/>
<encryption format='luks'>
  <secret type='passphrase' uuid='4ff78f8c-6ee6-4a8d-b638-2b59d5d49279'/>
  <cipher name='twofish' size='256' mode='cbc' hash='sha256'/>
  <ivgen name='plain64' hash='sha256'/>
</encryption>
</target>
</volume>

And I try to create this volume in the rbd pool, got error:
# virsh vol-create rbd rbd_vol1.xml 
error: Failed to create vol from rbd_vol1.xml
error: unsupported configuration: storage pool does not support encrypted volumes




Actual results:
vol-create failed

Expected results:
should support 

=======Additional info=======
we can use pure qemu cmd to create a rbd vol with luks encryption as follow:
# qemu-img create -f luks --object secret,id=sec0,data=`printf %s "redhat" | base64`,format=base64 -o key-secret=sec0 rbd:yisun-pool/ys2.img:mon_host=10.73.75.52 1G
Formatting 'rbd:yisun-pool/ys2.img:mon_host=10.73.75.52', fmt=luks size=1073741824 key-secret=sec0


# qemu-img info rbd:yisun-pool/ys2.img:mon_host=10.73.75.52
image: rbd:yisun-pool/ys2.img:mon_host=10.73.75.52
file format: luks
virtual size: 1.0G (1073741824 bytes)
disk size: unavailable
encrypted: yes