rust: add qemu-api doctests to "meson test"

rust: add qemu-api doctests to "meson test"

Doctests are weird.  They are essentially integration tests, but they're
"ran" by executing rustdoc --test, which takes a compiler-ish
command line.  This is supported by Meson 1.8.0.

Because they run the linker and need all the .o files, run them in the
build jobs rather than the test jobs.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>