qemu: Change default machine type for RISC-V

qemu: Change default machine type for RISC-V

It's quite difficult, if not impossible, to create a working RISC-V VMs
using the current default machine type of 'spike_v1.10'. Change the
default to the more appropriate and virtualization friendly 'virt'
machine type.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>