Introduce new XMLs to specify disk source using libvirt storage

Introduce new XMLs to specify disk source using libvirt storage

With this patch, one can specify the disk source using libvirt
storage like:

  <disk type='volume' device='disk'>
    <driver name='qemu' type='raw' cache='none'/>
    <source pool='default' volume='fc18.img'/>
    <target dev='vdb' bus='virtio'/>
  </disk>

"seclabels" and "startupPolicy" are not supported for this new
disk type ("volume"). They will be supported in later patches.

docs/formatdomain.html.in:
  * Add documents for new XMLs
docs/schemas/domaincommon.rng:
  * Add rng for new XMLs;
src/conf/domain_conf.h:
  * New struct for 'volume' type disk source (virDomainDiskSourcePoolDef)
  * Add VIR_DOMAIN_DISK_TYPE_VOLUME for enum virDomainDiskType
src/conf/domain_conf.c:
  * New helper virDomainDiskSourcePoolDefParse to parse the 'volume'
    type disk source.
  * New helper virDomainDiskSourcePoolDefFree to free the source def
    if 'volume' type disk.
tests/qemuxml2argvdata/qemuxml2argv-disk-source-pool.xml:
tests/qemuxml2xmltest.c:
  * New test