Fix Test failure TestAccLibvirtVolume_RepeatedName
Fix Test failure TestAccLibvirtVolume_RepeatedName

Fix https://github.com/dmacvicar/terraform-provider-libvirt/issues/730
by making volumeDelete return nil if
client.libvirt.LookupStorageVolByKey returns an ERR_NO_STORAGE_VOL
error.  This is in the same vein as recent changes made by @moio in
commit 4d5aa53fb.

dmacvicar/terraform-provider-libvirt,dmacvicar/terraform-provider-libvirt,dmacvicar/terraform-provider-libvirt