hw/arm/virt: Make EL2 accelerator check an accept-list

hw/arm/virt: Make EL2 accelerator check an accept-list

Currently only the TCG and qtest accelerators can handle an EL2
guest.  Instead of making the condition check be "fail if KVM or HVF"
(an exclude-list), make it a be "allow if TCG or qtest" (an
accept-list).

This is better for if/when we add new accelerators, as it makes the
default be that we forbid an EL2 guest.  This is the most likely to
be correct and also "fails safe"; if the new accelerator really can
support EL2 guests then the implementor will see that they need to
add it to the accept-list.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20250623121845.7214-20-philmd@linaro.org
[PMM: rewrote commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>