btrfs/003: enable test with virtio_blk devices in VM

btrfs/003: enable test with virtio_blk devices in VM

For a long time this test has been failing on all kinds of VM
configuration, which are using virtio_blk devices. This is due to
the fact that scsi devices are deletable and virtio_blk are not.
However, this only prevents device replace case to run and has no
negative effect on the other useful test cases.

Re-enable btrfs/003 to run by making
_require_deletable_scratch_dev_pool private to the test case and
modifying it to return success (0) or failure (1) if devices are not
deletable. Further modify the replace test case to check the return
value of this function and skip it if devices are not deletable.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>