Use the stored job name rather than passing in the disk alias when
referring to the job which allows the same code to work also when
-blockdev will be used.

Note that this API does not require the change to use 'query-job' as it
will ever only work with blockjobs bound to disks due to the arguments
which allow only referring to a disk. For the disk-less jobs we'll need
to add a separate API later.

The change to qemuMonitorGetBlockJobInfo is required as the API was
stripping the 'drive-' prefix when returning the data which is not
desired any more.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>