docs: compiling: Separate 'prepare', 'configure', and 'build' steps

docs: compiling: Separate 'prepare', 'configure', and 'build' steps

Only the preparation of sources differs between a build from a git
checkout vs a build from tarball. Restructure the docs to outline the
difference and combine information on how to configure libvirt.

Most notably the suggestion to use '-Dsystem=true' was present only for
the steps to build a git checkout.

Suggest also running the testsuite as part of the build step.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>