qmp: add query-cpus-fast

The query-cpus command has an extremely serious side effect:
it always interrupts all running vCPUs so that they can run
ioctl calls. This can cause a huge performance degradation for
some workloads. And most of the information retrieved by the
ioctl calls are not even used by query-cpus.

This commit introduces a replacement for query-cpus called
query-cpus-fast, which has the following features:

 o Never interrupt vCPUs threads. query-cpus-fast only returns
   vCPU information maintained by QEMU itself, which should be
   sufficient for most management software needs

 o Drop "halted" field as it can not be retrieved in a fast
   way on most architectures

 o Drop irrelevant fields such as "current", "pc" and "arch"

 o Rename some fields for better clarification & proper naming
   standard

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>
Message-Id: <1518797321-28356-3-git-send-email-mihajlov@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>