qapi: add query-cpu-definitions command (v2)

This command attempts to map to the behavior of -cpu ?.  Unfortunately, the
output of this command differs wildly across targets.

To accommodate this, we use a weak symbol to implement a default version of the
command that fails with a QERR_NOT_SUPPORTED error code.  Targets can then
override and implement this command if it makes sense for them.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
