<pool type='logical'>
	<source>
		<name>MyVG</name>
		<device name='/dev/sdb'/>
...

However, dumping out the XML for this same storage pool (with, say, virsh
pool-dumpxml), gives:

<pool type='logical'>
	<source>
		<name>MyVG</name>
		<device name='/dev/sdb'>
		</device>


To make this more idempotent, do the <device name='/dev/sdb'/> form by default,
and only do the <device>...</device> form if .nfreeExtent is defined for the
storage pool.
Signed-off-by: NChris Lalancette <clalance@redhat.com>