storage: lvm: lvcreate fails with allocation=0, don't do that

storage: lvm: lvcreate fails with allocation=0, don't do that

On F17 at least, this command fails:

$ sudo /usr/sbin/lvcreate --name sparsetest -L 0K --virtualsize 16384K vgvirt
  Unable to create new logical volume with no extents

Which is unfortunate since allocation=0 is what virt-manager tries to use
by default.

Rather than telling the user 'don't do that', let's just give them the
smallest allocation possible if alloc=0 is requested.

https://bugzilla.redhat.com/show_bug.cgi?id=866481