rbd: use 'rbd ls' without '-l' to find free names

rbd: use 'rbd ls' without '-l' to find free names

with more than a few images, 'rbd ls -l' gets rather slow
compared to a simple 'rbd ls'. since we only need to check
existing image names for finding a free one, the latter is
sufficient.

example with ~400 rbd images:
$ time rbd ls -p ceph-vm > /dev/null
real    0m0.027s
user    0m0.012s
sys     0m0.008s

$ time rbd ls -l -p ceph-vm > /dev/null
real    0m5.250s
user    0m1.632s
sys     0m0.584s

a linked clone of two disks on the same setup accordingly
also shows a massive speedup:

$ time qm clone 1000 10000 -snap test
create linked clone of drive scsi0 (ceph-vm:vm-1000-disk-2)
clone vm-1000-disk-2: vm-1000-disk-2 snapname test to
vm-10000-disk-1
create linked clone of drive scsi1 (ceph-vm:vm-1000-disk-1)
clone vm-1000-disk-1: vm-1000-disk-1 snapname test to
vm-10000-disk-2

real    0m11.157s
user    0m3.752s
sys     0m1.308s

$ time qm clone 1000 10000 -snap test
create linked clone of drive scsi1 (ceph-vm:vm-1000-disk-1)
clone vm-1000-disk-1: vm-1000-disk-1 snapname test to
vm-10000-disk-1
create linked clone of drive scsi0 (ceph-vm:vm-1000-disk-2)
clone vm-1000-disk-2: vm-1000-disk-2 snapname test to
vm-10000-disk-2

real    0m0.872s
user    0m0.652s
sys     0m0.096s

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>