virsysinfo: Parse OEM strings

virsysinfo: Parse OEM strings

Setting OEM strings for a domain was introduced in
v4.1.0-rc1~315. However, any application that wanted to use them
(e.g. to point to an URL where a config file is stored) had to
'dmidecode -u --oem-string N' (where N is index of the string).
Well, we can expose them under our <sysinfo/> XML and if the
domain is running Libvirt inside it can be obtained using
virConnectGetSysinfo() API.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>