qemu: Add --enable-slirp to force checking libslirp

It's recommended and I guess most people will need it, so let's be
explicit and prevent some user errors.

postlfs/virtualization/qemu.xml

@@ -232,6 +232,7 @@ cd        build &&
              --target-list=$QEMU_ARCH    \
              --audio-drv-list=alsa       \
              --disable-pa                \
+             --enable-slirp              \
              --docdir=/usr/share/doc/qemu-&qemu-version; &&
 
 unset QEMU_ARCH &&
@@ -296,6 +297,12 @@ chmod 4750 /usr/libexec/qemu-bridge-helper</userinput></screen>
       sets the audio driver to ALSA. See below for enabling other audio drivers.
     </para>
 
+    <para>
+      <parameter>--enable-slirp</parameter>: This switch forces the building
+      system to check for <xref linkend='libslirp'/>.  Remove it if you
+      don't need the <option>-netdev user</option> support.
+    </para>
+
     <para>
       <option>--audio-drv-list=pa --disable-alsa</option>: This switch sets
       the audio driver to pulseaudio. For other drivers see the