qemuimage-testlib: kill qemu process according to its pid, instead of process name
poky-qemu-internal will set up a tap lockfile when creating tap device. The lockfile
will be released when a TERM signal is received. In previous code, function
Test_Kill_Qemu uses pkill to kill all process named "qemu". This may cause lockfile
release function not work in poky-qemu-internal. Then poky-qemu-internal will be
hang when user start QEMU the second time. To prevent the issue, the new function
Test_Kill_Qemu kills all child pid with a given parent process ID.
