riscv64: backport HiFive unleashed & cloud-image building support
All commits are cherrypicks from hirsute, and includes:
make-lxd-metadata: add riscv64 lxd architecture tag
control: install qemu-utils & snapd on riscv64.
disk-image.binary: actually skip building MBR image on EFI platforms.
disk-image.binary: skip building MBR image on riscv64.
disk-image.binary: however still build MBR image on amd64.
disk-image-uefi.binary: build riscv64 image without a bootloader.
disk-image-uefi: riscv64 add u-boot spl
disk-image-uefi: riscv64 add ubuntu:ubuntu login, without expiry.
riscv64: build preinstalled riscv64 image with uboot SPL and CIDATA.
riscv64: fixup subarch build, unbound variable
qcow2-image: unbreak builds without subarch.
disk-image-uefi: fix riscv64 subarch user-data.
LP: #1903034