virtio_mmio: Support command-line parameters

The Virtio MMIO bus driver was added in 2014 with support for devices
exposed via FDT; in 2018 support was added to discover Virtio MMIO
devices via ACPI tables, as in QEMU. The Firecracker VMM eschews both
FDT and ACPI, instead presenting device information via kernel command
line arguments of the form virtio_mmio.device=<parameters>.

These command line parameters get converted into kernel environment
variables; this adds support for parsing those variables and attaching
virtio_mmio children to nexus.

There is a case to be made that it would be cleaner to have a new
"cmdlinebus" attached to nexus and virtio_mmio children attached to
that. A future commit might do that.

Discussed with: imp, jrtc27
Sponsored by: https://patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D36189