configure: Don't add Xen's libs to LDFLAGS

configure: Don't add Xen's libs to LDFLAGS

When Xen is detected via pkg-config, it isn't necessary to modify
LDFLAGS as modifying libs_softmmu is enough.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>