Resolve mounts host_path relatively to vmtest.toml

When a non-default `rootfs` is used, and vmtest is run through
`go test -exec`, the binary executed within the VM can't be found,
because that rootfs doesn't contain the test binary compiled on the fly.
So a custom mount has to be specified.

But, before that commit, if a relative `host_path` was used, it'd be
passed as is to qemu, and thus would be resolved as relative to the cwd
where qemu was running.

In the case of Go, if a directory was passed to `go test` (eg.
`./integration/...`), and `GOTMPDIR=.` was set, Go would compile the
test binary within its original cwd (eg. project root dir), and then
launch vmtest in the 'directory under test' (eg. `./integration`), so
the relative mount would resolve to the directory where vmtest was
started (eg. `./integration`) -- a directory that doesn't contain the
test binary.

To fix this mismatch, resolve `host_path` relative to the `vmtest.toml`
file.

FWIW this doesn't fully solve the original issue, unless the VM path
put in the config file is hardcoded to the cwd of `go test -exec`. To
overcome this, one can prepend the inline command with a call to
`mkdir` + `ln`:

```sh
GOTMPDIR=. go test -exec "vmtest -c $(pwd)/vmtest.toml -- mkdir $(realpath ..); ln -s /skbdump/ $(pwd); " ./integration/...
```

Signed-off-by: Albin Kerouanton <albinker@gmail.com>

docs/config.md

@@ -86,6 +86,8 @@ The Mount struct for defining additional host mounts into the VM.
 * `host_path` (string)
     * Required field
     * Path on the host.
+    * If a relative path is provided, it will be interpreted as relative to
+      `vmtest.toml`
 * `writable` (bool)
     * Optional field
     * Whether this mount is writable in the VM.

src/vmtest.rs

@@ -119,6 +119,16 @@ impl Vmtest {
         target.kernel = target.kernel.map(|s| self.resolve_path(s.as_path()));
         target.rootfs = self.resolve_path(target.rootfs.as_path());
         target.vm.bios = target.vm.bios.map(|s| self.resolve_path(s.as_path()));
+        target.vm.mounts = target
+            .vm
+            .mounts
+            .iter()
+            .map(|(k, v)| {
+                let mut m = v.clone();
+                m.host_path = self.resolve_path(v.host_path.as_path());
+                (k.clone(), m)
+            })
+            .collect();
 
         Qemu::new(updates, target, &self.base).context("Failed to setup QEMU")
     }