hyperv/vmbus: avoid crash, panic if vbe fb info is missing

Do not assume that VBE framebuffer metadata can be used. Like with the
EFI fb metadata, it may be null, so we should take care not to
dereference the null vbefb pointer. This avoids a panic when booting
-CURRENT on a gen1 VM in Azure.

Approved by: tsoome
Sponsored by: Miles AS
Differential Revision: https://reviews.freebsd.org/D27533