qga: vss-win32: Fix interference with snapshot deletion by other VSS request

qga: vss-win32: Fix interference with snapshot deletion by other VSS request

When a VSS requester such as vshadow.exe or diskshadow.exe requests to
delete snapshots, qemu-ga VSS provider's DeleteSnapshots() is also called
and returns E_NOTIMPL, that makes the deletion fail.
To avoid this issue, return S_OK and set values that represent no snapshots
are deleted by qemu-ga VSS provider.

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Reviewed-by: Gal Hammer <ghammer@redhat.com>
Reviewed-by: Yan Vugenfirer <yvugenfi@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
(cherry picked from commit d9e1f574cb6eac0a3a2f97b67d2e7a3ad9c1dc95)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>