target/arm/hvf: Release memory allocated by hv_vcpu_config_create()

target/arm/hvf: Release memory allocated by hv_vcpu_config_create()

hv_vcpu_config_create() is documented in <Hypervisor/hv_vcpu_config.h>
as:

  /*!
   @abstract Creates a vcpu configuration object.
   @result A new vcpu configuration object. This should be released with os_release when no longer used.
   */
  OS_OBJECT_RETURNS_RETAINED OS_WARN_RESULT
  hv_vcpu_config_t hv_vcpu_config_create(void);

Release the memory allocated by hv_vcpu_config_create() with
os_release().

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Mads Ynddal <mads@ynddal.dk>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>