configure: convert accelerator variables to meson options

Prepare for moving the tests to meson.  For now they only have
enabled/disabled as the possible values when meson is invoked,
but "auto" will be a possibility later, when configure will only
parse the command line options.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>