tools/libxc: Use featuresets rather than guesswork

tools/libxc: Use featuresets rather than guesswork

It is conceptually wrong to base a VM's featureset on the features visible to
the toolstack which happens to construct it.

Instead, the featureset used is either an explicit one passed by the
toolstack, or the default which Xen believes it can give to the guest.

Collect all the feature manipulation into a single function which adjusts the
featureset, and perform deep dependency removal.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>