vhost: fix vm_wait_for_boot arg check.

This was changed recently to enable a shell coding style checker, but $*
does not represent the number of arguments supplied to the function,
rather it is an alternative version of $@ that stores all arguments in a
single string. So we should be checking against an empty string and not
0.

By checking against 0, every time we passed 0 into vm_wait_for_boot, we
tried booting every vm in that directory. This causes errors on nightly
tests and intermittent failures during per-patch testing.

Change-Id: I1850ea73ea59fdc244de1a91e4c6de1b4505e518
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/470087


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>