xl: make libxl_uuid2string internal to libxenlight

xl: make libxl_uuid2string internal to libxenlight

libxenlight exports a function libxl_uuid2string which is used
internally in several places but has one external caller in xl.
This means that libxl internal callers leak since they were not
expecting to have to free() the UUID since the per-api-call-gc-lifetime
patch.

Convert libxl_uuid2string to be an internal function which participates
in the callers garbage collection. Eliminate string_of_uuid() macro in
favour of "format" and "arguments" macros suitable for printf()-like
functions which are made part of the libxl API and fix-up xl callers to
use that to avoid code duplication and enhance readability.

Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
committer: Stefano Stabellini <stefano.stabellini@eu.citrix.com>