rbd: bump the default scratch size for xfstests to 10G

autobuild-ceph.git commit 53db7a34aba5 had silently changed the default
elevator from cfq to deadline, which made xfstests 167 very unhappy.
It looks like with deadline and noop elevators it requires a ~6G
scratch partition.  Bump the default scratch image size to 10G.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>