tests: add ivshmem qtest

Adds 4 ivshmemtests:
- single qemu instance and basic IO
- pair of instances, check memory sharing
- pair of instances with server, and MSIX
- hot plug/unplug

A temporary shm is created as well as a directory to place server
socket, both should be clear on exit and abort.

Cc: Cam Macdonell <cam@cs.ualberta.ca>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
