cpu: Convert CpuInfo into flat union

cpu: Convert CpuInfo into flat union

The CpuInfo struct is used only by the 'query-cpus' output
command, so we are free to modify it by adding fields (clients
are already supposed to ignore unknown output fields), or by
changing optional members to mandatory, while still keeping
QMP wire compatibility with older versions of qemu.

When qapi type CpuInfo was originally created for 0.14, we had
no notion of a flat union, and instead just listed a bunch of
optional fields with documentation about the mutually-exclusive
choice of which instruction pointer field(s) would be provided
for a given architecture.  But now that we have flat unions and
introspection, it is better to segregate off which fields will
be provided according to the actual architecture.  With this in
place, we no longer need the fields to be optional, because the
choice of the new 'arch' discriminator serves that role.

This has an additional benefit: the old all-in-one struct was
the only place in the code base that had a case-sensitive
naming of members 'pc' vs. 'PC'.  Separating these spellings
into different branches of the flat union will allow us to add
restrictions against future case-insensitive collisions, since
that is generally a poor interface practice.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1447836791-369-25-git-send-email-eblake@redhat.com>
[Spelling of CPUInfo{SPARC,PPC,MIPS} fixed]
Signed-off-by: Markus Armbruster <armbru@redhat.com>