Implement SHM region management and re-enable DAX in virtiofs

[slp]

We have multiple devices (fs and gpu) that can use SHM regions. So far,
we were creating a single SHM region so only one device could make use
of it. In this commit, we implement SHM region management so multiple
devices can request their own regions.

For the moment, we're hardcoding the SHM region sizes for gpu and fs. A
future commit will extend the API so users can configure those sizes as
desired.

[slp]

cc/ @AsahiLina

[rhatdan]

@tylerfanelli Can you review this?

[tylerfanelli]

@slp Any way I can test?

[slp]

@tylerfanelli yes, with this commit in and building with gpu, in the guest you should see:

• virtio-gpu gets a memory window:

[    1.191469] [drm] Host memory window: 0x280000000 +0x200000000

• each virtio-fs device gets its own DAX window:

[    1.982987] virtiofs virtio4: Cache len: 0x40000000 @ 0x700000000
[    1.989999] virtiofs virtio5: Cache len: 0x40000000 @ 0x740000000

• mounting the virtio-fs volume with DAX should work (even on macOS!), and reading files should be possible:

slp@localhost:~$ sudo mount -t virtiofs -o dax test /mnt
slp@localhost:~$ dd if=/mnt/mistral-7b-instruct-v0.2.Q4_0.gguf of=/dev/null bs=128k
31348+1 records in
31348+1 records out
4108917024 bytes (4.1 GB, 3.8 GiB) copied, 1.23938 s, 3.3 GB/s

[slp]

Let's put this on hold until we figure out what's happing in #213 (comment)

[slp]

@tylerfanelli PTAL, should be ready now.

[tylerfanelli]

LGTM on x86_64 Linux

@slp Side note, with this built and latest version of libkrunfw, the chroot_vm example is failing for me (x86_64 Linux). I'll submit an issue so we can track this. Can you confirm?

[slp]

@tylerfanelli yes, please, open an issue detailing that.

[tylerfanelli]

Tracked with #216