Add memory mapping support with KVM

[jprendes]

This PR adds memory mapping with KVM.
This means that we are now only missing support for memory mapping on Windows.

It also adds a smoke test for the supported platforms.

[jprendes]

I tried making the test a bit more complex, where the guest tries to access an mmap buffer that hasn't been mapped by the host.
That guest function call fails with Err(Error("MMIO access address 0x100000000")) as expected, but then the sandbox seem to be left in a bad state even after the state is restored from the last snapshot.

Subsequent call (a simple PrintOutput call in this case) fails with

Err(GuestAborted(8, "panicked at /home/jprendes/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/flatbuffers-25.2.10/src/endian_scalar.rs:142:5:\ninsufficient capacity for emplace_scalar, needed 1 got 0"))

I think there must be some sandbox state we aren't correctly restoring after a guest call fails with certain modes.

[syntactically]

LGTM