bhyve: read out graphics stolen memory address and size

[corvink]

Summary

This is the first step to emulate the graphics stolen memory register.

Note that the graphics stolen memory is somehow confusing. On the one
hand the Intel Open Source HD Graphics Programmers' Reference Manual
states that it's only GPU accessible. As the CPU can't access the area,
the guest shouldn't need it. On the other hand, the Intel GOP driver
refuses to work properly, if it's not set to a proper address.

Intel itself maps it into the guest by EPT [1]. At the moment, we're not
aware of any situation where this EPT mapping is required, so we don't
do it yet.

Intel also states that the Windows driver for Tiger Lake reads the
address of the graphics stolen memory [2]. As the GVT-d code doesn't
support Tiger Lake in it's first implementation, we can't check how it
behaves. We should keep an eye on it.

[1]
https://github.com/projectacrn/acrn-hypervisor/blob/e28d6fbfdfd556ff1bc3ff330e41d4ddbaa0f897/devicemodel/hw/pci/passthrough.c#L655-L657
[2]
https://github.com/projectacrn/acrn-hypervisor/blob/e28d6fbfdfd556ff1bc3ff330e41d4ddbaa0f897/devicemodel/hw/pci/passthrough.c#L626-L629

[corvink]

It's required to properly emulate the graphics stolen memory register. Therefore, this is the first patch require to do so.

[markj]

usr.sbin/bhyve/pci_gvt-d.c

81

87

errno can be clobbered by the warnx() call above. I think it's enough to just return -1. Then you don't need errno.h.