Instead of having a generic "virt_qemu" board use "virt_qemu_<arch>" in
order to have a clean distinction between boards. Current supported
boards are "virt_qemu_arm_v7a", "virt_qemu_arm_v8a", and
"virt_qemu_riscv".
issue #4034
* Add new virtio device model
* Extend test run-script with vfat block test image
* Add vmm depot src recipe
* Use packages in test run-script
Fix#4025
* Remove SPEC declarations from mk/spec
* Remove all board-specific REQUIRE declaratiions left
* Replace [have_spec <board>] run-script declarations with have_board where necessary
* Remove addition of BOARD variable to SPECS in toplevel Makefile
* Move board-specific directories in base-hw out of specs
* Fix GIC model to support priority and cpu target settings correspondingly
* Fix semantic of SGIR register for GICv2
* Minor GIC model IRQ state fix
* Introduce synchronization for VirtIO and GIC models
* Enable multiple CPUs in test run-script for ARMv8
Fix#3926
