driver: Introduce global driver feature flag handling function

driver: Introduce global driver feature flag handling function

The 'virDrvFeature' has a combination of features which are asserted by
the specific driver and features which are actually global.

In many cases the implementation was cargo-culted into newer drivers
without re-assesing whether it makes sense.

This patch introduces a global function which will specifically handle
these global flags and defer the rest to the driver.

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