hw/i386: Introduce the microvm machine type

hw/i386: Introduce the microvm machine type

microvm is a machine type inspired by Firecracker and constructed
after its machine model.

It's a minimalist machine type without PCI nor ACPI support, designed
for short-lived guests. microvm also establishes a baseline for
benchmarking and optimizing both QEMU and guest operating systems,
since it is optimized for both boot time and footprint.

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