Turn QEMU capabilities object into a full virObjectPtr

Turn QEMU capabilities object into a full virObjectPtr

The current qemu capabilities are stored in a virBitmapPtr
object, whose type is exposed to callers. We want to store
more data besides just the flags, so we need to move to a
struct type. This object will also need to be reference
counted, since we'll be maintaining a cache of data per
binary. This change introduces a 'qemuCapsPtr' virObject
class. Most of the change is just renaming types and
variables in all the callers

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>