tests/qtest/migration-test: Fix bootfile cleanup handling

tests/qtest/migration-test: Fix bootfile cleanup handling

If you invoke the migration-test binary in such a way that it doesn't run
any tests, then we never call bootfile_create(), and at the end of
main() bootfile_delete() will try to unlink(NULL), which is not valid.
This can happen if for instance you tell the test binary to run a
subset of tests that turns out to be empty, like this:

 (cd build/asan && QTEST_QEMU_BINARY=./qemu-system-x86_64 ./tests/qtest/migration-test --tap -k -p bang)
 # random seed: R02S6501b289ff8ced4231ba452c3a87bc6f
 # Skipping test: userfaultfd not available
 1..0
 ../../tests/qtest/migration-test.c:182:12: runtime error: null pointer passed as argument 1, which is declared to never be null
 /usr/include/unistd.h:858:48: note: nonnull attribute specified here

Handle this by making bootfile_delete() not needing to do anything
because bootfile_create() was never called.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
[fixed conflict with aee07f2563]
Signed-off-by: Fabiano Rosas <farosas@suse.de>