qemu: Convert aborting of a domain job to a switch statement

qemu: Convert aborting of a domain job to a switch statement

Until now we only really aborted migration via qemuDomainAbortJob. This
will change with the upcoming addition of the backup job. Additionally
there were a bunch of if statements checking various aspects of the
current job.

To make it more obvious convert qemuDomainAbortJob to use a switch
statement and move the individual conditions to the appropriate job
type.

Every job type has now it's own case despite multiple job types just
plainly cancelling the job for clarity and future extension.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>