fix storage volume creation error messages
fix storage volume creation error messages

It seems that raising of error messages associated with creation of
storage volumes contained copy'n'paste errors due to the code being
copied from network creation code.

Signed-off-by: Adam Spiers <a3b1ac66d44425905084924ee6cef90b31d2e761@suse.com>

libvirt/ruby-libvirt,libvirt/ruby-libvirt,libvirt/ruby-libvirt