incremental_backup: Fix backup job not canceled issue

[hs0210]

Based on #3625, genereate 1000M data to slow down backup process.

Before:

# /usr/local/bin/avocado run --vt-type libvirt --test-runner=runner --vt-machine-type arm64-mmio incremental_backup.push_mode.original_disk_local.coldplug_disk.backup_to_raw.backup_to_block.negative_test.
destroy_vm --vt-connect-uri qemu:///system 
JOB ID     : 7c24f41ead0d1ef8af78c55e43e4e31f61cfbedd                                                                                                                                                                                        
JOB LOG    : /var/lib/avocado/job-results/job-2022-12-05T04.29-7c24f41/job.log                                                                                                                                                               
 (1/1) type_specific.io-github-autotest-libvirt.incremental_backup.push_mode.original_disk_local.coldplug_disk.backup_to_raw.backup_to_block.negative_test.destroy_vm:  FAIL: Backup job should be canceled but not. (58.63 s)               
RESULTS    : PASS 0 | ERROR 0 | FAIL 1 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/lib/avocado/job-results/job-2022-12-05T04.29-7c24f41/results.html
JOB TIME   : 59.00 s

After:

# /usr/local/bin/avocado run --vt-type libvirt --test-runner=runner --vt-machine-type arm64-mmio incremental_backup.push_mode.original_disk_local.coldplug_disk.backup_to_raw.backup_to_block.negative_test.
destroy_vm --vt-connect-uri qemu:///system                                                                     
JOB ID     : 98858526f36f5ada5418ca065ec41c1d056dc9ab
JOB LOG    : /var/lib/avocado/job-results/job-2022-12-05T04.43-9885852/job.log
 (1/1) type_specific.io-github-autotest-libvirt.incremental_backup.push_mode.original_disk_local.coldplug_disk.backup_to_raw.backup_to_block.negative_test.destroy_vm: PASS (72.87 s)                                                        
RESULTS    : PASS 1 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/lib/avocado/job-results/job-2022-12-05T04.43-9885852/results.html
JOB TIME   : 73.24 s

Signed-off-by: Hu Shuai [email protected]