qcow2: Fix error handling in the compression code

This patch fixes a few things in the way error codes are handled in
the qcow2 compression code:

a) qcow2_co_pwritev_compressed() expects qcow2_co_compress() to only
   return -1 or -2 on failure, but this is not correct. Since the
   change from qcow2_compress() to qcow2_co_compress() in commit
   ceb029cd the new code can also return -EINVAL (although
   there does not seem to exist any code path that would cause that
   error in the current implementation).

b) -1 and -2 are ad-hoc error codes defined in qcow2_compress().
   This patch replaces them with standard constants from errno.h.

c) Both qcow2_compress() and qcow2_co_do_compress() return a negative
   value on failure, but qcow2_co_pwritev_compressed() stores the
   value in an unsigned data type.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>