When docs are explicitly requested, require Sphinx>=1.6.0. When docs are
explicitly disabled, don't bother to check for Sphinx at all. If docs
are set to "auto", attempt to locate Sphinx, but continue onward if it
wasn't located.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-22-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>