[Libguestfs] Create new Sys::VirtV2V::Util

[Matthew Booth]

These 2 patches are mostly code motion. They were prompted by an apparent augeas
error in BZ 613967 which didn't display useful error message. The error seems to
happen in Converter::Linux. GuestOS::RedHat had a handy function which displayed
verbose augeas error messages. This function moves into the new module where it
can be used by both modules.

The second patch is an consequential tidy up. UserMessage moves into the new
module.

* [PATCH 1/2] Move augeas error reporting into new Sys::VirtV2V::Util
* [PATCH 2/2] Move user_message into Sys::VirtV2V::Util