accel/kvm: Declare kvm_direct_msi_allowed in stubs

accel/kvm: Declare kvm_direct_msi_allowed in stubs

Avoid when calling kvm_direct_msi_enabled() from
arm_gicv3_its_common.c the next commit:

  Undefined symbols for architecture arm64:
    "_kvm_direct_msi_allowed", referenced from:
        _its_class_name in hw_intc_arm_gicv3_its_common.c.o
  ld: symbol(s) not found for architecture arm64

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230405160454.97436-3-philmd@linaro.org>