rbd: make default size larger for xfstests
rbd: make default size larger for xfstests

Test 167 runs out of space on newer kernels

Signed-off-by: Josh Durgin <51652c783f70f5b272ebd02f1fd20d3af7a2449f@inktank.com>

ktdreyer/teuthology,dmick/teuthology,tchaikov/teuthology,ivotron/teuthology,SUSE/teuthology,caibo2014/teuthology,t-miyamae/teuthology,zhouyuan/teuthology,dmick/teuthology,ktdreyer/teuthology,ceph/teuthology,tchaikov/teuthology,yghannam/teuthology,dreamhost/teuthology,caibo2014/teuthology,zhouyuan/teuthology,michaelsevilla/teuthology,robbat2/teuthology,SUSE/teuthology,dmick/teuthology,yghannam/teuthology,SUSE/teuthology,michaelsevilla/teuthology,ceph/teuthology,robbat2/teuthology,dreamhost/teuthology,ivotron/teuthology,t-miyamae/teuthology