sysinfo: define internal driver API

sysinfo: define internal driver API

* src/driver.h (virDrvGetSysinfo): New typedef.
(_virDriver): New callback member.
* src/esx/esx_driver.c (esxDriver): Add stub for driver.
* src/lxc/lxc_driver.c (lxcDriver): Likewise.
* src/opennebula/one_driver.c (oneDriver): Likewise.
* src/openvz/openvz_driver.c (openvzDriver): Likewise.
* src/phyp/phyp_driver.c (phypDriver): Likewise.
* src/qemu/qemu_driver.c (qemuDriver): Likewise.
* src/remote/remote_driver.c (remote_driver): Likewise.
* src/test/test_driver.c (testDriver): Likewise.
* src/uml/uml_driver.c (umlDriver): Likewise.
* src/vbox/vbox_tmpl.c (Driver): Likewise.
* src/vmware/vmware_driver.c (vmwareDriver): Likewise.
* src/xen/xen_driver.c (xenUnifiedDriver): Likewise.
* src/xenapi/xenapi_driver.c (xenapiDriver): Likewise.