tests: Wait explicitly for vm shutdown w/ the 'wait-timeout' crate

Instead of waiting blindly with fixed amount of sleeping time, we can
use the `wait-timeout` crate to explicitly wait VM shutdown (with a
timeout). It can reduces the execution time of some tests
substantially. Also, this patch increases the `shutdown` timeout for
'test_reboot', which should fix the recent sporadic failures on this
test.

Signed-off-by: Bo Chen <chen.bo@intel.com>

Cargo.lock

@@ -234,6 +234,7 @@ dependencies = [
  "vhost_user_net",
  "vmm",
  "vmm-sys-util",
+ "wait-timeout",
 ]
 
 [[package]]
@@ -1676,6 +1677,15 @@ dependencies = [
  "serde_derive",
 ]
 
+[[package]]
+name = "wait-timeout"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9f200f5b12eb75f8c1ed65abd4b2db8a6e1b138a20de009dacee265a2498f3f6"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "wasi"
 version = "0.9.0+wasi-snapshot-preview1"