rrdp_iostat: push the exception handler for listing a domain's VBDs inwards

No functional change; refactoring only.
CA-141867: Look in both /local/domain/0/backend/vbd and vbd3 when listing a domain's VBDs

tapdisk2 uses vbd, tapdisk3 uses vbd3, so the domain's VBDs could be in either.

(We might have looked up the correct backend path for each VBD from the frontend
info, but we can't trust that a malicious frontend might not manipulate that
info.)
Merge pull request #10 from jjd27/CA-141867-v2

CA-141867 (v2): backend paths may be either /local/domain/0/device/vbd or vbd3