virsh: Refactor parseRateStr to avoid false-positive uninitialized variable

virsh: Refactor parseRateStr to avoid false-positive uninitialized variable

Commit 6983d6d2 tried to improve parseRateStr but broke the build
instead for compilers that were not able to properly introspect the for
loop indexed by the enum resulting into the following error:

virsh-domain.c: In function 'parseRateStr':
virsh-domain.c:916:13: error: 'field_name' may be used uninitialized in this function [-Werror=maybe-uninitialized]
             vshError(ctl, _("malformed %s field"), field_name);
             ^
virsh-domain.c:915:13: error: 'tmp' may be used uninitialized in this function [-Werror=maybe-uninitialized]
         if (virStrToLong_ullp(token, NULL, 10, tmp) < 0) {
             ^
Rather than trying to fix the code, refactor the function again by
reusing virStringSplit.