[xen-tools] lvcreate not found on fedora

Hi,
On Fedora, lvcreate (and other lvm tools) are placed in /usr/sbin,
while xen-tools expects them to be in /sbin - that breaks lvm installs

When there'd be something in the Makefile that sets theses pathes,
probing for /usr/sbin and /sbin,  one could check it and give Fedora
users an out-of-the box working xen-tools install. (apart from other
problems I'll hopefully not find :) )

Henning