coverity: provide Coverity-friendly MIN_CONST and MAX_CONST

coverity: provide Coverity-friendly MIN_CONST and MAX_CONST

Coverity has problems seeing through __builtin_choose_expr, which
result in it abandoning analysis of later functions that utilize a
definition that used MIN_CONST or MAX_CONST, such as in qemu-file.c:

 50    DECLARE_BITMAP(may_free, MAX_IOV_SIZE);

CID 1429992 (#1 of 1): Unrecoverable parse warning (PARSE_ERROR)1.
expr_not_constant: expression must have a constant value

As has been done in the past (see 07d66672), it's okay to dumb things
down when compiling for static analyzers.  (Of course, now the
syntax-checker has a false positive on our reference to
__COVERITY__...)

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Fixes: CID 1429992, CID 1429995, CID 1429997, CID 1429999
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200629162804.1096180-1-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>