Fix QEMU driver custom domain status XML extensions

Fix QEMU driver custom domain status XML extensions

Invoking the virConnectGetCapabilities() method causes the QEMU
driver to rebuild its internal capabilities object. Unfortunately
it was forgetting to register the custom domain status XML hooks
again.

To avoid this kind of error in the future, the code which builds
capabilities is refactored into one single method, which can be
called from all locations, ensuring reliable rebuilds.

* src/qemu/qemu_driver.c: Fix rebuilding of capabilities XML and
  guarentee it is always consistent