vnc: Hoist allocation of VncBasicInfo to callers

A future qapi patch will rework generated structs with a base
class to be unboxed.  In preparation for that, change the code
that allocates then populates an info struct to instead merely
populate the fields of an info field passed in as a parameter
(renaming vnc_basic_info_get* to vnc_init_basic_info*). Add
rudimentary Error handling at the lowest levels for cases
where the old code returned NULL; but rather than plumb Error
all the way through the stack, the callers drop the error and
return NULL as before.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1445898903-12082-7-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>