sysinfo: expose new API

sysinfo: expose new API

The new virConnectGetSysinfo() API allows one to get the system
information associated to a connection host, providing the same
data as a guest that uses <os><smbios mode='host'/></os>, and in
a format that can be pasted into the guest and edited when using
<os><smbios mode='sysinfo'/></os>.

* include/libvirt/libvirt.h.in (virConnectGetSysinfo): Declare.
* src/libvirt_public.syms: Export new symbol.