OvmfPkg/BhyvePkg: reserve igd memory by E820

[corvink]

Summary

Detecting the Graphics Stolen Memory and OpRegion address and length is
platform specific. OVMF shouldn't be platform specific. Therefore, move
the platform specific part to bhyve. Bhyve detects the Graphics Stolen
Memory and OpRegion address and length and passes these information as
E820 table to the firmware. The firmware then picks up the E820 table
and reserves all memory regions accordingly. This approach allow us to
pass other platform specific memory ranges to the guest without any
firmware modifications in the future too.