qemu: Add support for controling qemu capabilities via the qemu XML namespace

qemu: Add support for controling qemu capabilities via the qemu XML namespace

Similarly how we allow adding arbitrary command line arguments and
environment variables this patch introduces the ability to control
libvirt's perception of the qemu process by tweaking the capability bits
for testing purposes.

The idea is to allow developers and users either test a new feature by
enabling it early or disabling it to see whether it introduced
regressions.

This feature is not meant for production use though, so users should
handle it with care.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>