The behavior of --{enable,disable}-pypi is similar to that of
-Dwrapmode={default,nodownload} respectively.  In particular,
in both cases a feature needs to be explicitly enabled for the
dependency to be downloaded.

So, use a single option to control both cases.  Now, --enable-slirp
will trigger cloning and building of libslirp if the .pc file
is not found on the machine.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>