softmmu: Use accel_find("xen") instead of xen_available()

The xen_available() function is used only to produce an error
for some Xen-specific command line options in QEMU binaries where
Xen support was not compiled in: it just returns the value of
the CONFIG_XEN define.

Now that accelerators are QOM classes, we can check for
"does this binary have Xen compiled in" with accel_find("xen"),
and drop the xen_available() function.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210730105947.28215-2-peter.maydell@linaro.org