configure: stop user enabling plugins on Windows for now

There are some patches on the list that enable plugins on Windows but
they still need some changes to be ready:

  https://patchew.org/QEMU/20201013002806.1447-1-luoyonggang@gmail.com/



In the meantime lets stop the user from being able to configure the
support so they don't get confused by the weird linker error messages
later.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Cc: Yonggang Luo <luoyonggang@gmail.com>
Message-Id: <20210709143005.1554-32-alex.bennee@linaro.org>