sanitycheck: Fix incorrect QEMUHandler status reporting.

The current QEMUHandler implementation in sanitycheck does not check
for the process exit code and reports "PASS" even when either the QEMU
executable cannot be launched or exited immediately due to an error
(e.g. unsupported machine type, missing file, ...).

This commit adds QEMU process exit code check and error reporting when
the exit code is a non-zero value.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>