Use generic SYSCTL_* macro instead of deprecated SYSCTL_VNET_*.

Suggested by:	glebius
xen: don't set suspend/resume methods for the PIRQ PIC

The suspend/resume of event channels is already handled by the xen_intr_pic.
If those methods are set on the PIRQ PIC they are just called twice, which
breaks proper resume. This fix restores migration of FreeBSD guests to a
working state.

Sponsored by: Citrix Systems R&D