block: Clean up "Could not create temporary overlay" error message

bdrv_create() sets an error and returns -errno on failure.  When the
latter is interesting, the error is created with error_setg_errno().

bdrv_append_temp_snapshot() uses the error's message to create a new
one with error_setg_errno().  This adds a strerror() that is either
uninteresting or duplicate.  Use error_setg() instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1450452927-8346-7-git-send-email-armbru@redhat.com>