virsh-pool.c: Don't jump over variable declaration

virsh-pool.c: Don't jump over variable declaration

Since 785ff34bf8 we are using the outputStr variable in cleanup label.
However, there is a possibility to jump to the label before the variable
has been declared:

virsh-pool.c: In function 'cmdPoolList':
virsh-pool.c:1121:25: error: jump skips variable initialization [-Werror=jump-misses-init]
                         goto asprintf_failure;
                         ^
virsh-pool.c:1308:1: note: label 'asprintf_failure' defined here
 asprintf_failure:
 ^
virsh-pool.c:1267:11: note: 'outputStr' declared here
     char *outputStr = NULL;