vmctl(8): avoid abort when given an invalid "kernel" file.

vmctl(8): avoid abort when given an invalid "kernel" file.

The vmctl `start` command allows the user to pass an optional
"kernel" file (either a ramdisk kernel or a SeaBIOS image). This
file is opened by vmctl and the descriptor passed via imsg.

If the file provided isn't a regular file, the attempt to send the
start message to vmd(8)'s control socket will fail and results in
a the vmctl process aborting.

Check the file type after open and fail gracefully if not a regular
file.

reported by and ok gnezdo@