bhyve: add helper struct for qemus acpi table loader

The hypervisor is aware of all system properties. For the guest bios
it's hard and complex to detect all system properties. For that reason,
it would be better if the hypervisor creates acpi tables instead of the
guest. Therefore, the hypervisor has to send the acpi tables to the
guest. At the moment, bhyve just copies the acpi tables into the guest
memory. This approach has some restrictions. You have to keep sure that
the guest doesn't overwrite them accidentally. Additionally, the size of
acpi tables is limited.

Providing a plain copy of all acpi tables by fwcfg isn't possible. Acpi
tables have to point to each other. So, if the guest copies the acpi
tables into memory by it's own, it has to patch the tables. Due to
different layouts for different acpi tables, there's no generic way to
do that. For that reason, qemu created a table loader interface. It
contains commands for the guest for loading specific blobs into guest
memory and patching those blobs.

This commit adds a qemu_loader class which handles the creation of qemu
loader commands. At the moment, the WRITE_POINTER command isn't
implement. It won't be required by bhyve's acpi table generation yet.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D38438