Documentation: fix qemu build instructions

The current instructions fail during the qemu configure step, since meson
is not able to find IGVM. This is because meson searches for IGVM via
pkgconfig, so supply the location of the IGVM installation via
`PKGCONFIG_PATH`.

Signed-off-by: Carlos López <clopez@suse.de>

Author: 00xc

Documentation/docs/installation/INSTALL.md

@@ -113,7 +113,10 @@ Now the right branch is checked out and you can continue with the build.
 Feel free to adapt the installation directory to your needs:
 
 ```
-$ CONFIG_IGVM=$HOME/igvminst ./configure --prefix=$HOME/bin/qemu-svsm/ --target-list=x86_64-softmmu --enable-igvm
+$ PKGCONFIG_PATH="${PKGCONFIG_PATH}:$HOME/igvminst/usr/lib/x86_64-linux-gnu/pkgconfig/" ./configure \
+    --prefix=$HOME/bin/qemu-svsm/ \
+    --target-list=x86_64-softmmu \
+    --enable-igvm
 $ C_INCLUDE_PATH=$HOME/igvminst/usr/include/ LIBRARY_PATH=$HOME/igvminst/usr/lib/x86_64-linux-gnu/ ninja -C build/
 $ make install
 ```