qemu: Infrastructure for better support for non-x86

• build-qemu: Include 'none' in choices

  Fixes: 49a1aee1 "autopkgtest-build-qemu: Generalize --efi into --boot="

• build-qemu: Update man page

• qemu: Make the qemu architecture a first-class option

  Instead of guessing what qemu implementation we are using from the qemu command, make the dpkg and qemu architectures first-class options.

  If the qemu architecture is explictly given, it is used; otherwise it is derived from the dpkg architecture if given, or derived from dpkg --print-architecture on the host, or guessed from the qemu command for backwards compatibility, or guessed from uname -m on the host.

  If the qemu command is explicitly given, it is used; otherwise it is derived from the qemu architecture.

• qemu: Only -enable-kvm if VM is compatible with host architecture

  Loosely based on a patch from Ryutaroh Matsumoto.

  Note that ARM-specific and aarch64-specific quirks regarding machine and CPU selection are not included here: this is only the framework.

  (See !118 (merged) for ARM-specifics)

• virt-qemu: Generalize --efi into --boot, similar to build-qemu