meson: Improve network driver enablement logic

meson: Improve network driver enablement logic

The Homebrew package explicitly enables this driver despite us
disabling it by default on macOS, so it must be functional to
at least some extent and certainly can't be causing any build
failures.

Additionally, if the user has explicitly asked for the network
driver to be enabled but libvirtd is disabled for whatever
reason, we should error out instead of silently disabling the
network driver.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>