meson: sphinx-build

For now, sphinx is run on every invocation of make.  The previous mechanism
using $(wildcard) is not reproducible in Meson and was also brittle; for
example some .rst.inc files were left out.  The next patch will introduce
a Sphinx extension to emit a depfile.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>