hyperv/vmbus: Reorganize vmbus device tree
For GEN1 Hyper-V, vmbus is attached to pcib0, which contains the
resources for PCI passthrough and SR-IOV. There is no
acpi_syscontainer0 on GEN1 Hyper-V.
For GEN2 Hyper-V, vmbus is attached to acpi_syscontainer0, which
contains the resources for PCI passthrough and SR-IOV. There is
no pcib0 on GEN2 Hyper-V.
The ACPI VMBUS device now only holds its _CRS, which is empty as
of this commit; its existence is mainly for upward compatibility.
Device tree structure is suggested by jhb@.
MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D10565