[roll] Roll fuchsia [ffx][emu][pbm] Add multiple virtual device specs

This CL is addressing the issue of needing to have a more
resource constrained virtual device specification for running the
emulator on lower end machines and VMs.

There is room for improvement on the structure, naming and flexibility,
but this CL does the following:

1. Demonstrate how to add an additional virtual device spec to the PBM.
   This unfortunately is still the same for all products, but should be
   an improvement over the single definition.
2. Updates `ffx emu` with a `--device` option to select the virtual
   device spec to use. A value of `list` will list the available names.
   If not specified, it uses the first device in the list.

Usage:
`--device list` will list the available devices:

```
$ ffx emu start
file:///usr/local/google/home/wilkinsonclay/tq/fuchsia/out/default/gen/build/images/*.json#workstation.qemu-x64 \
--device list
Valid virtual device specifications are: ["qemu-x64-emu-min",
"qemu-x64-emu-recommended"]
```

Then run the emulator:

ffx emu start workstation.qemu-x64 --device qemu-x64-emu-min --headless
--engine qemu

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/682024
Original-Revision: efaed2f1061a1fccff79480d1008a0950890f30a
GitOrigin-RevId: 2331007e0f05118bd19f14f1f7a11421cf75e8fd
Change-Id: If1e0053980cb808fd3a42508b38452a097ac3d6d