Remove import xen.lowlevel.xc
Remove import xen.lowlevel.xc

There is still use of it: QubesHost.get_free_xen_memory and
QubesHost.measure_cpu_usage. Will migrate them to libvirt later (for now
some things will be broken - namely qubes-manager).

QubesOS/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin