virt-win-reg: Support HKEY_USERS keys.

virt-win-reg: Support HKEY_USERS keys.

This adds support for various Registry keys under HKEY_USERS (user
preferences).

(1) HKEY_USERS\<SID>
    where <SID> is a User SID.

For example:

  # virt-win-reg Windows 'HKEY_USERS\S-1-5-19'

lists out the LocalService user's registry.

(2) HKEY_USERS\<username>
    where <username> is a Windows local username (this is a
    libguestfs extension).

For example:

  # virt-win-reg Windows 'HKEY_USERS\rjones'

lists out the user preferences of user 'rjones'.

HKU can be used as an abbreviation for HKEY_USERS.  Merging is also
supported.