add support for snapshot disk source

[nanli1]

Need disk source in snapshot xml
Signed-off-by: nanli nanli@redhat.com
For autotest/tp-libvirt#5584

[cliping]

LGTM

[chloerh]

Seems Disk class already has 'source', is it possible that we don't need to add it again?

[nanli1]

Seems Disk class already has 'source', is it possible that we don't need to add it again?

hi haijiao ,Thanks your comment .This is for disk > source tag is in snapshot xml instead of disk xml ,Did I get your point

# cat s1.xml
<domainsnapshot>
<description>Snapshot test</description>
<name>s1</name>
<memory snapshot='no'/>
<disks>
<disk name='vda' snapshot='no'/>
<disk name='vdb' snapshot='external'>
<source file="/var/lib/libvirt/images/vdb.s1"/>
</disk>
</disks>
</domainsnapshot>

[nanli1]

Haijiao has helped to check that we have inherit disk class , no need to give this support