Rather than directly modifying fields in the qemuBlockJobDataPtr
structure add a bunch of fields which allow to do the transitions.

This will help later when adding more complexity to the job handling.

APIs introduced in this patch are:

qemuBlockJobDiskNew - prepare for starting a new blockjob on a disk
qemuBlockJobDiskGetJob - get the block job data structure for a disk

For individual job state manipulation the following APIs are added:
qemuBlockJobStarted - Sets the job as started with qemu. Until that
                      the job can be cancelled without asking qemu.

qemuBlockJobStartupFinalize - finalize job startup. If the job was
                              started in qemu already, just releases
                              reference to the job object. Otherwise
                              clears everything as if the job was never
                              started.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>