qemu: stop creating capabilities at driver startup

Now that nearly all internal APIs use the QEMU capabilities or other
QEMU driver data directly, there's no compelling benefit to create
virCapsPtr at driver startup.

Skipping this means we don't probe capabilities for all 30 system
emulator targets at startup, only those emulators which are referenced
by an XML doc. This massively improves libvirtd startup time when the
capabilities cache is not populated. It even improves startup time
when the cache is up to date, as we don't bother to load files from
the cache until we need them.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>