Use ocaml-xen-lowlevel-libs-devel instead of xen-ocaml-devel

Other packages are already using ocaml-xen-lowlevel-libs-devel,
and xen-ocaml-devel conflicts with this.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #5 from johnelse/fix-xen-libs-conflict

Use ocaml-xen-lowlevel-libs-devel instead of xen-ocaml-devel