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

No functional change; refactoring only.

(cherry picked from commit 51be2a8498d1aebb5aa70c21d8a4cb42de0ae898)

Imported-by: John Else <john.else@citrix.com>

Conflicts:
	rrdp_iostat.ml
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.)

(cherry picked from commit b8d0fa35c4db464ac7b25a88d6f6dbb9183246f5)

Imported-by: John Else <john.else@citrix.com>

Conflicts:
	rrdp_iostat.ml