snapshots: find max value currently used
snapshots: find max value currently used

The MAX() statement consistently returns snap9 when it should actualy return
snap10 or greater.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

Skarlso/lxd,Skarlso/lxd,atxwebs/lxd,stgraber/lxd,lxc/lxd,pcdummy/lxd,stgraber/lxd,stgraber/lxd,atxwebs/lxd,Skarlso/lxd,erikmack/lxd,jsavikko/lxd,mjeanson/lxd,lxc/lxd,georghopp/lxd,hallyn/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,techtonik/lxd,Skarlso/lxd,hallyn/lxd,lxc/lxd,techtonik/lxd,georghopp/lxd,atxwebs/lxd,mjeanson/lxd,georghopp/lxd,stgraber/lxd,hallyn/lxd,techtonik/lxd,pcdummy/lxd,erikmack/lxd,stgraber/lxd,erikmack/lxd,jsavikko/lxd,pcdummy/lxd,lxc/lxd,lxc/lxd,mjeanson/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,jsavikko/lxd,atxwebs/lxd