To allow the compile to detect incorrect printf formats, any
var-args function should have a format annotation
* common/macros.h: Helper to define ATTR_PRINTF for code
which can't depend on glib
* common/canvas_base.c, common/lz.h, common/macros.h: Annotate
some var-args methods
Be more library friendly, by not aborting in library errors.
spice_common now includes a proper log handler that will abort by
default when reaching a warning.
SPICE_ABORT_LEVEL can be changed to modify run-time abort level.
SPICE_DEBUG_LEVEL can be changed to be more verbose. By default, only
log level more importants than WARNING.
Only memory-related functions are allowed to abort(), since they are
not recoverable errors in the library.
