qga-win: Fix guest-get-fsinfo multi-disks collection

qga-win: Fix guest-get-fsinfo multi-disks collection

When a volume has more than one disk, all disks cannot be
returned correctly because there is not enough malloced memory
for disk extents, so before executing DeviceIoControl for the
second time, get the correct size of the required memory space
to store all disk extents.

Details:
https://learn.microsoft.com/en-us/windows/win32/api/winioctl/ns-winioctl-volume_disk_extents

Signed-off-by: Peng Ji <peng.ji@smartx.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>