oVirt wants to reuse the SPICE Guest Tools code as a basis for building
an installer for the oVirt Guest Tools. This commit renames the install
script to win-guest-tools.nsis and add conditionals to build either an
installer for SPICE Guest Tools or oVirt Guest Tools.

Change-Id: I62e41f3b3bcc5d1975523614897928543c44ba1a
Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>