Include virtio support in std.MALTA

[arichardson]

Summary

The MALTA kernel config is generally used for QEMU and having support
for VirtIO there by default is quite useful.

Test Plan

Used to work for CheriBSD, currently doesn't compile since D23401 landed.

[jrtc27]

sys/mips/conf/std.MALTA

64

MALTA doesn't have MMIO, so this really doesn't belong. In some ways neither does VirtIO in general, but PCI I guess is fine because that's not tied to the platform.

65

Canonical name is "VirtIO Entropy device" (see sys/{amd64,i386,riscv}/conf/NOTES).

66

This isn't widely used, we don't need this, and anyone who does can load it as a module late in boot.

67

Stray newline

[jhb]

I'm not sure it makes sense to commit this until it works?

[arichardson]

In D25217#559400, @jhb wrote:

I'm not sure it makes sense to commit this until it works?

I agree, I mostly posted this diff so that the regression in https://reviews.freebsd.org/D23401 can be debugged.

[arichardson]

address review comments

[arichardson]

D25416 has been committed. Any objections to me committing this?

[brooks]

LGTM