tools: hv: use getmntent in hv_vss_daemon

As suggested by Paolo Bonzini, use getmntent instead of parsing output
of mount(1).

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>