tests/qtest/migration: Move common test code

tests/qtest/migration: Move common test code

The migration tests have a set of core infrastructure routines. These
are functions that are called by (almost) all tests and centralize the
common operations of: starting migration on both sides, waiting for
guests to boot, performing guest initialization and teardown, guest
memory validation, etc.

Move this basic framework code (and a few static helpers) into a
separate file. Leave only individual test functions (and their own
static helpers) in migration-test.c.

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