conf: Refactor storage of guest capabilities

conf: Refactor storage of guest capabilities

The capabilities are declared in the XML schema so passing feature names
as strings from hypervisor drivers makes no sense.

Additionally some of the features expose so called 'toggles' while
others not. This knowledge was encoded by a bunch of 'STREQ's in the
formatter.

Change all of this by declaring the features as an enum and use it
instead of a dynamically allocated array.

Presence of 'toggles' is encoded together with the conversion strings
rather than in the formatter directly.

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