hmp.c: Revert hmp_info_cpus output format change

hmp.c: Revert hmp_info_cpus output format change

Commit 137b5cb6 refactored 'info cpus' output, changing
'thread_id' to 'thread-id'.  While HMP is not a stable
interface, it is trivial to keep the spelling consistent
for test frameworks that have not yet updated to using QMP.

This patch just reverts back output format to 'thread_id'.

CC: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>
Signed-off-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Message-Id: <20180327123800.28851-1-sathnaga@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
[eblake: improve commit message]
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>