The previous patch hoisted some bounds checks to the callers;
but someone that is not aware of the hoisted check could now
try passing an integer between LLONG_MAX and ULLONG_MAX.  As a
safety measure, add new json conversion modes that let libvirt
error out early instead of pass bad numbers to qemu, if the
caller ever makes a mistake due to later refactoring.

Convert the various blockjob QMP calls to use the new modes,
and switch some of them to be optional (QMP has always supported
an omitted "speed" the same as "speed":0, for everything except
block-job-set-speed).

* src/qemu/qemu_monitor_json.c (qemuMonitorJSONMakeCommandRaw):
Add 'j'/'y' and 'J'/'Y' to error out on negative input.
(qemuMonitorJSONDriveMirror, qemuMonitorJSONBlockCommit)
(qemuMonitorJSONBlockJob): Use it.
Signed-off-by: NEric Blake <eblake@redhat.com>