qemu-img: print short help on getopt failure

Printing the full help output obscures the error message for an invalid
command-line option or missing argument.

Before this patch:

  $ ./qemu-img --foo
  ...pages of output...

After this patch:

  $ ./qemu-img --foo
  qemu-img: unrecognized option '--foo'
  Try 'qemu-img --help' for more information

This patch adds the getopt ':' character so that it can distinguish
between missing arguments and unrecognized options.  This helps provide
more detailed error messages.

Suggested-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170317104541.28979-4-stefanha@redhat.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>