block/file-posix: fix g_file_get_contents return path

block/file-posix: fix g_file_get_contents return path

The g_file_get_contents() function returns a g_boolean. If it fails, the
returned value will be 0 instead of -1. Solve the issue by skipping
assigning ret value.

This issue was found by Matthew Rosato using virtio-blk-{pci,ccw} backed
by an NVMe partition e.g. /dev/nvme0n1p1 on s390x.

Signed-off-by: Sam Li <faithilikerun@gmail.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20230727115844.8480-1-faithilikerun@gmail.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>