ivshmem has its very own code to create and map shared memory.
Replace that with an implicitly created memory backend.  Reduces the
number of ways we create BAR 2 from three to two.

The memory-backend-file is currently available only with CONFIG_LINUX,
so this adds a second Linuxism to ivshmem (the other one is eventfd).
Should we ever need to make it portable to systems where
memory-backend-file can't be made to serve, we could create a
memory-backend-shmem that allocates memory with shm_open().

Bonus fix: shared memory files are now created with permissions 0655
instead of 0777.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <1458066895-20632-32-git-send-email-armbru@redhat.com>