Move the code for handling the QEMU virDomainObjPtr private
data, and custom XML namespace into a separate file

* src/qemu/qemu_domain.c, src/qemu/qemu_domain.h: New file
  for private data & namespace code
* src/qemu/qemu_driver.c, src/qemu/qemu_driver.h: Remove
  private data & namespace code
* src/qemu/qemu_driver.h, src/qemu/qemu_command.h: Update
  includes
* src/Makefile.am: Add src/qemu/qemu_domain.c