roms: add microvm-bios (qboot) as binary and git submodule

roms: add microvm-bios (qboot) as binary and git submodule

qboot is a minimalist x86 firmware for booting Linux kernels. It does
the mininum amount of work required for the task, and it's able to
boot both PVH images and bzImages without relying on option roms.

This characteristics make it an ideal companion for the microvm
machine type.

Signed-off-by: Sergio Lopez <slp@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>