lguest: fix early_ioremap.

dmi_scan_machine breaks under lguest:
	lguest: unhandled trap 14 at 0xc04edeae (0xffa00000)

This is because we use current_cr3 for the read_cr3() paravirt
function, and it isn't set until the first cr3 change.  We got away
with it until this happened.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>