qemu-io: Return non-zero exit code on failure

The result of openfile was not checked, leading to failure deep in the
actual command with confusing error message, and exiting with exit code 0.

Here is a simple example - trying to read with the wrong format:

    $ touch file
    $ qemu-io -f qcow2 -c 'read -P 1 0 1024' file; echo $?
    can't open device file: Image is not in qcow2 format
    no file open, try 'help open'
    0

With this patch, we fail earlier with exit code 1:

    $ ./qemu-io -f qcow2 -c 'read -P 1 0 1024' file; echo $?
    can't open device file: Image is not in qcow2 format
    1

Failing earlier, we don't log this error now:

    no file open, try 'help open'

But some tests expected it; the line was removed from the test output.

Signed-off-by: Nir Soffer <nirsof@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20170201003120.23378-2-nirsof@gmail.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
