qemu: Add a shortcut for running tests on an EFI-booted image (bug#898340)

Some Debian derivatives strongly prefer EFI bootloaders (SteamOS and Apertis are examples), some "legacy-free" disk image preparation tools like mkosi can only use EFI boot, and in distributions that support both BIOS and EFI it is useful to be able to exercise both code paths.

qemu can be provided with EFI boot capabilities by plugging in OVMF (x86) or AAVMF (arm/aarch64) firmware, both from src:edk2, as an emulated parallel flash device. The syntax isn't completely obvious, so an --efi shortcut seems useful to have.

Signed-off-by: Simon McVittie smcv@collabora.com Bug: https://bugs.debian.org/898340