vl: fix breakage of -tb-size

Commit e7b161d573 ("vl: add tcg_enabled() for tcg related code") adds
a check to exit the program when !tcg_enabled() while parsing the -tb-size
flag.

It turns out that when the -tb-size flag is evaluated, tcg_enabled() can
only return 0, since it is set (or not) much later by configure_accelerator().

Fix it by unconditionally exiting if the flag is passed to a QEMU binary
built with !CONFIG_TCG.

Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
