hw/arm/virt: Simplify logic for setting instance's 'tcg_its' variable

hw/arm/virt: Simplify logic for setting instance's 'tcg_its' variable

Because 'tcg_its' in the machine instance is set based on the machine
class’s negated variable 'no_tcg_its', 'tcg_its' is the opposite of
'no_tcg_its' and hence the code in question can be simplified as:
tcg_its = !no_tcg_its.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-id: 20250628195722.977078-3-gustavo.romero@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>