fix(riscv/sbi): populate return value field

This commit ensures the return value is written to the a1 vCPU register.
Previously, the register was always written with an uninitialized value,
often defaulting to zero.

Signed-off-by: João Peixoto <[email protected]>

Author: joaopeixoto13

src/arch/riscv/sbi.c

@@ -405,6 +405,7 @@ static struct sbiret sbi_bao_handler(unsigned long fid)
     struct sbiret ret;
 
     ret.error = hypercall(fid);
+    ret.value = (long int)vcpu_readreg(cpu()->vcpu, REG_A1);
 
     return ret;
 }