Coverity fix with vmi_get_bit

CID 703173 (#1 of 1): Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)overflow_before_widen: Potentially overflowing expression 1 << bit with type int (32 bits, signed) is evaluated using 32-bit arithmetic, and then used in a context that expects an expression of type reg_t (64 bits, unsigned).