tests/qtest/migration: Add a precopy file test with fdset

tests/qtest/migration: Add a precopy file test with fdset

Add a test for file migration using fdset. The passing of fds is more
complex than using a file path. This is also the scenario where it's
most important we ensure that the initial migration stream offset is
respected because the fdset interface is the one used by the
management layer when providing a non empty migration file.

Note that fd passing is not available on Windows, so anything that
uses add-fd needs to exclude that platform.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>