qemu-ga: adding vss-[un]install options

Existing command line options include '-s install' and '-s uninstall'.
These options install/uninstall both Windows QEMU GA service
and optional VSS COM server. The QEMU GA Windows service allows
always-on serving guest agent's QMP commands and VSS COM server
enables guest agent integration with Volume Shadow Service.

This commit introdices new options '-s vss-install' and '-s vss-uninstall',
affecting only GA VSS COM server registration. The new options are useful
for registering and unregistering the COM server during MSI installation,
upgrade and uninstallation.

Signed-off-by: Yossi Hindin <yhindin@redhat.com>
Message-Id: <1430913460-13174-2-git-send-email-yhindin@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>