qcow2: Fix warnings in check_refcount() (Kevin Wolf)

qcow2: Fix warnings in check_refcount() (Kevin Wolf)

From: Kevin Wolf <kwolf@redhat.com>

This code is currently only compiled when DEBUG_ALLOC is defined, so you
usually don't see compiler warnings on it. This patch series wants to enable
the code, so fix the format string warnings first.

While we're at it, let's print error messages to stderr.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7213 c046a42c-6fe2-441c-8c8c-71466251a162