Add new domain device: "controller"

Add new domain device: "controller"

This augments virDomainDevice with a <controller> element
that is used to represent disk controllers (e.g., scsi
controllers). The XML format is given by

  <controller type="scsi" index="<num>">
     <address type="pci" domain="0xNUM" bus="0xNUM" slot="0xNUM"/>
  </controller>

where type denotes the disk interface (scsi, ide,...), index
is an integer that identifies the controller for association
with disks, and the <address> element specifies the controller
address on the PCI bus as described in previous commits
The address element can be omitted; in this case, an address
will be assigned automatically.

Most of the code in this patch is from Wolfgang Mauerer's
previous disk controller series

 * docs/schemas/domain.rng: Define syntax for <controller>
   XML element
 * src/conf/domain_conf.c, src/conf/domain_conf.h: Define
   virDomainControllerDef struct, and routines for parsing
   and formatting XML
* src/libvirt_private.syms: Add virDomainControllerInsert
   and virDomainControllerDefFree