In cases when e.g. a new feature breaks upstream behaviour it's useful
to allow users to disable the new feature to verify the regression and
possibly use it as a workaround until a fix is available.

The new qemu.conf option named "capability_filters" allows to remove
qemu capabilities from the detected bitmap.

This patch introduces the configuration infrastructure to parse the
option and pass it around.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>