tools/libxc: Shuffle definitions and uses of min()/max() macros

tools/libxc: Shuffle definitions and uses of min()/max() macros

Move the typesafe min() macro from xc_dom_decompress_unsafe_xz.c to
xc_private.h, and complement it with an equivalent max() macro.

Replace current users of type unsafe MIN()/MAX() macros, and remove their
scattered definitions.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>