network: try to eliminate default network conflict during package install

Sometimes libvirt is installed on a host that is already using the
network 192.168.122.0/24. If the libvirt-daemon-config-network package
is installed, this creates a conflict, since that package has been
hard-coded to create a virtual network that also uses
192.168.122.0/24. In the past libvirt has attempted to warn of /
remediate this situation by checking for conflicting routes when the
network is started, but it turns out that isn't always useful (for
example in the case that the *other* interface/network creating the
conflict hasn't yet been started at the time libvirtd start its own
networks).

This patch attempts to catch the problem earlier - at install
time. During the %post install script for
libvirt-daemon-config-network, we use a case statement to look through
the output of "ip route show" for a route that exactly matches
192.168.122.0/24, and if found we search for a similar route that
*doesn't* match (e.g. 192.168.124.0/24) (note that the search starts
with "124" instead of 123 because of reports of people already
modifying their L1 host's network to 192.168.123.0/24 in an attempt to
solve exactly the problem we are also trying to solve).  When we find
an available route, we just replace all occurrences of "122" in the
default.xml that is being created with the newly found 192.168
subnet. This could obviously be made more complicated - examine the
template defaul.xml to automatically determine the existing network
address and mask rather than hard coding it in the specfile, etc, but
this scripting is simpler and gets the job done as long as we continue
to use 192.168.122.0/24 in the template. (If anyone with mad bash
skillz wants to suggest something to do that, by all means please do).

This is intended to at least "further reduce" occurrence of the
problems detailed in:

  https://bugzilla.redhat.com/show_bug.cgi?id=811967

(cherry picked from commit 5f719596)

libvirt.spec.in

@@ -1728,8 +1728,37 @@ fi
     %if %{with_network}
 %post daemon-config-network
 if test $1 -eq 1 && test ! -f %{_sysconfdir}/libvirt/qemu/networks/default.xml ; then
+    # see if the network used by default network creates a conflict,
+    # and try to resolve it
+    # NB: 192.168.122.0/24 is used in the default.xml template file;
+    # do not modify any of those values here without also modifying
+    # them in the template.
+    orig_sub=122
+    sub=${orig_sub}
+    nl='
+'
+    routes="${nl}$(ip route show | cut -d' ' -f1)"
+    case ${routes} in
+      *"${nl}192.168.${orig_sub}.0/24${nl}"*)
+        # there was a match, so we need to look for an unused subnet
+        for new_sub in $(seq 124 254); do
+          case ${routes} in
+          *"${nl}192.168.${new_sub}.0/24${nl}"*)
+            ;;
+          *)
+            sub=$new_sub
+            break;
+            ;;
+          esac
+        done
+        ;;
+      *)
+        ;;
+    esac
+
     UUID=`/usr/bin/uuidgen`
-    sed -e "s,</name>,</name>\n  <uuid>$UUID</uuid>," \
+    sed -e "s/${orig_sub}/${sub}/g" \
+        -e "s,</name>,</name>\n  <uuid>$UUID</uuid>," \
          < %{_datadir}/libvirt/networks/default.xml \
          > %{_sysconfdir}/libvirt/qemu/networks/default.xml
     ln -s ../default.xml %{_sysconfdir}/libvirt/qemu/networks/autostart/default.xml