qemu: Translate the iscsi pool/volume disk source

qemu: Translate the iscsi pool/volume disk source

The difference with already supported pool types (dir, fs, block)
is: there are two modes for iscsi pool (or network pools in future),
one can specify it either to use the volume target path (the path
showed up on host) with mode='host', or to use the remote URI qemu
supports (e.g. file=iscsi://example.org:6000/iqn.1992-01.com.example/1)
with mode='direct'.

For 'host' mode, it copies the volume target path into disk->src. For
'direct' mode, the corresponding info in the *one* pool source host def
is copied to disk->hosts[0].