Options such as "--enable-capstone=git" do not make much sense when building
from a tarball.  Accept "internal" for consistency with the meson options.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20211007130829.632254-17-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>