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@.

Tested-by:	dexuan@
Collabrated-wth:	dexuan@
Sponsored by:	Microsoft
Differential Revision:	https://reviews.freebsd.org/D10565