Make use of private data structure for monitor state

Make use of private data structure for monitor state

Introduce a new qemuDomainObjPrivate object which is used to store
the private QEMU specific data associated with each virDomainObjPtr
instance. This contains a single member, an instance of the new
qemuMonitorPtr object which encapsulates the QEMU monitor state.
The internals of the latter are private to the qemu_monitor* files,
not to be shown to qemu_driver.c

* src/qemu/qemu_conf.h: Definition of qemuDomainObjPrivate.
* src/qemu/qemu_driver.c: Register a functions for creating
  and freeing qemuDomainObjPrivate instances with the domain
  capabilities. Remove the qemudDispatchVMEvent() watch since
  I/O watches are now handled by the monitor code itself. Pass
  a new qemuHandleMonitorEOF() callback into qemuMonitorOpen
  to allow notification when the monitor quits.
* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h: Introduce
  the 'qemuMonitor' object. Temporarily add new APIs
  qemuMonitorWrite, qemuMonitorRead, qemuMonitorWaitForInput
  to allow text based monitor impl to perform I/O.
* src/qemu/qemu_monitor_text.c: Call APIs for reading/writing
  to monitor instead of accessing the file handle directly.