libxl: support dom0

libxl: support dom0

In Xen, dom0 is really just another domain that supports ballooning,
adding/removing devices, changing vcpu configuration, etc. This patch
adds support to the libxl driver for managing dom0. Note that the
legacy xend driver has long supported managing dom0.

Operations that are not supported on dom0 are filtered in libvirt
where a sensible error is reported. Errors from libxl are not
always helpful. E.g., attempting a save on dom0 results in

2015-06-23 15:25:05 MDT libxl: debug: libxl_dom.c:1570:libxl__toolstack_save: domain=0 toolstack data size=8
2015-06-23 15:25:05 MDT libxl: debug: libxl.c:979:do_libxl_domain_suspend: ao 0x7f7e68000b70: inprogress: poller=0x7f7e68000930, flags=i
2015-06-23 15:25:05 MDT libxl-save-helper: debug: starting save: Success
2015-06-23 15:25:05 MDT xc: detail: xc_domain_save_suse: starting save of domid 0
2015-06-23 15:25:05 MDT xc: error: Couldn't map live_shinfo (3 = No such process): Internal error
2015-06-23 15:25:05 MDT xc: detail: Save exit of domid 0 with errno=3
2015-06-23 15:25:05 MDT libxl-save-helper: debug: complete r=1: No such process
2015-06-23 15:25:05 MDT libxl: error: libxl_dom.c:1876:libxl__xc_domain_save_done: saving domain: domain did not respond to suspend request: No such process
2015-06-23 15:25:05 MDT libxl: error: libxl_dom.c:2033:remus_teardown_done: Remus: failed to teardown device for guest with domid 0, rc -8

Signed-off-by: Jim Fehlig <jfehlig@suse.com>