get_vmcs_field() returns a VMCS field offset which is normally unsigned, but it
also returns negatives (-1 here) on invalid arguments. Following the convention
we use for vectors, pins, etc., use a special unsigned value to indicate such
errors.
v1 -> v2:
* Use a special value (VMX_INVALID_VMCS_FIELD) instead of a seperate output
parameter to indicate errors.
Signed-off-by: Junjie Mao <junjie.mao@intel.com>