propolis-standalone: don't hang on ctrl-c before console is attached

[luqmana]

A long standing pet peeve of mine I finally got around to fixing:

Before:

$ pfexec ./target/release/propolis-standalone ~/VMs/testvm.toml
Feb 11 14:05:17.970 INFO Creating VM with 2 vCPUs, 536870912 lowmem, 0 highmem
Feb 11 14:05:18.129 INFO VM created, name: testvm
Feb 11 14:05:18.268 INFO Generating E820 map for guest address space
Feb 11 14:05:18.268 INFO Waiting for a connection to ttya
^CFeb 11 14:05:19.305 INFO Destroying instance...                       # Ctrl-C here
Feb 11 14:05:19.305 INFO Instance event ReqHalt (User("Ctrl+C"))
Feb 11 14:05:19.305 INFO State transition Initialize -> Quiesce
Feb 11 14:05:19.305 INFO State transition Quiesce -> Halt
Feb 11 14:05:19.305 INFO State transition Halt -> Destroy
Feb 11 14:05:19.306 INFO Instance destroyed

# Hangs here until there's a connection made to `ttya`

Feb 11 14:05:25.355 INFO Starting instance...

This is what the other end looks like:

$ sercons ./ttya

 * Connected.  Escape sequence is <Enter>#.

 * EOF on read.

After:

$ pfexec ./target/release/propolis-standalone ~/VMs/testvm.toml
Feb 11 14:13:58.434 INFO Creating VM with 2 vCPUs, 536870912 lowmem, 0 highmem
Feb 11 14:13:58.519 INFO VM created, name: testvm
Feb 11 14:13:58.534 INFO Generating E820 map for guest address space
Feb 11 14:13:58.534 INFO Waiting for a connection to ttya
^CFeb 11 14:13:59.494 INFO Destroying instance...
Feb 11 14:13:59.494 INFO Instance event ReqHalt (User("Ctrl+C"))
Feb 11 14:13:59.494 INFO State transition Initialize -> Quiesce
Feb 11 14:13:59.494 INFO State transition Quiesce -> Halt
Feb 11 14:13:59.518 INFO State transition Halt -> Destroy
Feb 11 14:13:59.518 INFO Instance destroyed

[jordanhendricks]

BLESS YOU

[jclulow]

See also, I guess: #562