xeniface: fixed context list cleanup
xeniface: fixed context list cleanup

QubesOS/qubes-vmm-xen-win-pvdrivers-xeniface,rneilturner/xeniface,QubesOS/qubes-vmm-xen-win-pvdrivers-xeniface,MarkKMueller/xeniface,rneilturner/xeniface,omeg/xeniface,MarkKMueller/xeniface,omeg/xeniface,MarkKMueller/xeniface,omeg/xeniface,rneilturner/xeniface,QubesOS/qubes-vmm-xen-win-pvdrivers-xeniface