qemu: blockjob: Refuse to register blockjob if disk already has one

qemu: blockjob: Refuse to register blockjob if disk already has one

Most code paths prevent starting a blockjob if we already have one but
the job registering function does not do this check. While this isn't a
problem for regular cases we had a bad test case where we registered two
jobs for a single disk which leaked one of the jobs. Prevent this in the
registering function until we allow having multiple jobs per disk.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
ACKed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>