Hyper-V: vmbus: Add NULL check for vmbus_res

QEMU emulates Hyper-V [1] but lacks the emulation for vmbus_res, thus no
coherence information is available. Add NULL check for it and fallback
to no coherence. This will prevent FreeBSD guests from panic on QEMU
with the Hyper-V enlightenment hv-synic enabled.

For real Hyper-V, both gen1 and gen2 have vmbus_res then they are not
affected by this change.

1. https://www.qemu.org/docs/master/system/i386/hyperv.html

PR: 274810
Reviewed by: mhorne, emaste, delphij, whu
Diagnosed by: mhorne
Fixes: e7a9817b8d32 Hyper-V: vmbus: implementat bus_get_dma_tag in vmbus
Insta-MFC approved by: re (delphij) for 14.0-RC4
Differential Revision: https://reviews.freebsd.org/D42414