spec: add dmidecode as prereq

https://bugzilla.redhat.com/show_bug.cgi?id=754909 complains that
because libvirt didn't require dmidecode, that the logs are noisy
and virConnectGetSysinfo needlessly fails.  Even 'virt-what' requires
dmidecode, so it's not that onerous of a dependency.  We may be
able to drop this in the future when we move to parsing sysfs data,
but for now, listing the dependency will help matters.

* libvirt.spec.in (Requires): Sort Requires before BuildRequires.
Add dmidecode.

libvirt.spec.in

@@ -332,6 +332,15 @@ Requires: device-mapper
 %if %{with_cgconfig}
 Requires: libcgroup
 %endif
+# For virConnectGetSysinfo
+Requires: dmidecode
+# For service management
+%if %{with_systemd}
+Requires(post): systemd-units
+Requires(post): systemd-sysv
+Requires(preun): systemd-units
+Requires(postun): systemd-units
+%endif
 
 # All build-time requirements
 BuildRequires: python-devel
@@ -482,13 +491,6 @@ BuildRequires: nfs-utils
 # Fedora build root suckage
 BuildRequires: gawk
 
-%if %{with_systemd}
-Requires(post): systemd-units
-Requires(post): systemd-sysv
-Requires(preun): systemd-units
-Requires(postun): systemd-units
-%endif
-
 %description
 Libvirt is a C toolkit to interact with the virtualization capabilities
 of recent versions of Linux (and other OSes). The main package includes