conf: Wire up the vhost-scsi connection from/to XML

conf: Wire up the vhost-scsi connection from/to XML

With the QEMU components in place, provide the XML parsing to
invoke that code when given the following XML snippet:

    <hostdev mode='subsystem' type='scsi_host'>
      <source protocol='vhost' wwpn='naa.501234567890abcd'/>
    </hostdev>

An optional address element can be specified within the hostdev
(pick CCW or PCI as necessary):

    <address type='ccw' cssid='0xfe' ssid='0x0' devno='0x0625'/>
    <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>

Add basic vhost-scsi tests which were cloned from hostdev-scsi-virtio-scsi
in both xml2argv and xml2xml. Added ones for both vhost-scsi-ccw and
vhost-scsi-pci since the syntaxes are slightly different between them.

Also adjusted the docs to describe the changes.

Signed-off-by: Eric Farman <farman@linux.vnet.ibm.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>