qemu: blockjob: Add job types for 'snapshot-save/delete'

qemu: blockjob: Add job types for 'snapshot-save/delete'

The snapshot creation/deletion QMP commands use the qemu 'job' API
to signal completion thus we need to add corresponding job types.

As the job handles everything internally we don't store anything about
the job.

Signed-off-by: Nikolai Barybin <nikolai.barybin@virtuozzo.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>