Introduce callbacks for serializing domain object private data to XML

Introduce callbacks for serializing domain object private data to XML

Now that drivers are using a private domain object state blob,
the virDomainObjFormat/Parse methods are no longer able to
directly serialize all neccessary state to/from XML. It is
thus neccessary to introduce a pair of callbacks fo serializing
private state.

The code for serializing vCPU PIDs and the monitor device
config can now move out of domain_conf.c and into the
qemu_driver.c where they belong.

* src/conf/capabilities.h: Add callbacks for serializing private
  state to/from XML
* src/conf/domain_conf.c, src/conf/domain_conf.h: Remove the
  monitor, monitor_chr, monitorWatch, nvcpupids and vcpupids
  fields from virDomainObjPtr. Remove code that serialized
  those fields
* src/libvirt_private.syms: Export virXPathBoolean
* src/qemu/qemu_driver.c: Add callbacks for serializing monitor
  and vcpupid data to/from XML
* src/qemu/qemu_monitor.h, src/qemu/qemu_monitor.c: Pass monitor
  char device config into qemuMonitorOpen directly.