qemu.tests: Fix bug that 'Tail' object has no attribute 'read_until_last_line_matches'

Now VM.process is a Tail object which do not have read_until_last_line_matches attribute.

So remove read_until_last_line_matches() function call.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.tests.migration_with_dst_problem: Check its status before start/stop a service.

Signed-off-by: Feng Yang <fyang@redhat.com>