qemu-img: Make resize error message more general

qemu-img: Make resize error message more general

The issue:

  $ qemu-img resize -f qcow2 foo.qcow2
  qemu-img: Expecting one image file name
  Try 'qemu-img --help' for more information

So we gave an image file name, but we omitted the length.  qemu-img
thinks the last argument is always the size and removes it immediately
from argv (by decrementing argc), and tries to verify that it is a valid
size only at a later point.

So we do not actually know whether that last argument we called "size"
is indeed a size or whether the user instead forgot to specify that size
but did give a file name.

Therefore, the error message should be more general.

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1523458
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20180205162745.23650-1-mreitz@redhat.com
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>