sysprep: Replace --user-accounts option with --{remove,keep}-user-accounts.

The --user-accounts option, with its double-negative '-' prefix on
user names, is confusing.  Replace it with '--remove-user-accounts'
and '--keep-user-accounts' options.

This updates commit 128d474095bfabc2d724d0181155e364f1afbabf.
sysprep: Add an optional method for checking if unused arguments were passed to disabled operations.
sysprep: Check --{keep,remove}-user-accounts parameters are not used when operation is disabled (RHBZ#1141157).

You will see an error like this:

$ virt-sysprep --remove-user-accounts foo,bar -a /dev/null
virt-sysprep: error: user-accounts: --remove-user-accounts parameter was
used, but the "user-account" operation is not enabled
v2v: -i libvirtxml: Give an error if local disks are not readable (RHBZ#1140946).
v2v: Add -o null mode.

In theory this discards the output.  Unfortunately in practice we have
to write the output to a temporary file and delete it (because of
limitations in qemu-img convert).
v2v: esx: Clarify that username can be specified in vpx:// URI (RHBZ#1141113).

If there is an authentication error when getting the session cookie,
emphasize in the error message that the username can be specified in
the URI.

Unfortunately libvirt asks for the username previously, but we cannot
access that.
sysprep: Ensure error handler surrounds all the code so we catch and print all errors.
sysprep: Use 'error' function consistently throughout the program.
Version 1.27.44.