tools/hv: Parse /etc/os-release

There is a new convention, used by systemd and supported by most
distributions, to put basic OS release information in /etc/os-release.
Added some additional error checking on strdup()

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>