qcow2GetFeatures: Extract population of features bitmap

qcow2GetFeatures: Extract population of features bitmap

Prepare for extraction of features from the 'incompatible features'
group.

This is done by moving the extraction loop into a new function called
qcow2GetFeaturesProcessGroup. The new function also allows to ingore
features we don't care about by passing VIR_STORAGE_FILE_FEATURE_LAST as
the target flag.

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