Remove bogus xen-devel dep from libvirt-devel RPM

The public libvirt API does not have any application visible
dependency on Xen libraries. The xen-devel dependency is thus
bogus
(cherry picked from commit 899bf668)
Signed-off-by: NCole Robinson <crobinso@redhat.com>

libvirt.spec.in

@@ -578,9 +578,6 @@ Summary: Libraries, includes, etc. to compile with the libvirt library
 Group: Development/Libraries
 Requires: %{name}-client = %{version}-%{release}
 Requires: pkgconfig
-%if %{with_xen}
-Requires: xen-devel
-%endif
 
 %description devel
 Includes and documentations for the C library providing an API to use