Configure option `--enable-plugins` makes modules in shared library not loadable on macOS
Description of problem:
The title mentions `--enable-plugins` option, however as it's enabled by default, not providing `--disable-plugins` would also cause this to happen.

If TCG plugin support is enabled, symbols in `qemu-system-*` binaries will be missing, and module libraries would fail to load as they expect those symbols to exist in the main binary.

Configure options used: `STRIP="strip -x" ./configure --enable-user --enable-tools --enable-parallels --enable-libxml2 --enable-spice --enable-hvf --enable-cocoa --enable-guest-agent --enable-curses --enable-plugins --enable-modules --objcc=gcc --enable-libusb --enable-usb-redir`

After inspecting the compiler command line, I've found the linker option `-Wl,-exported_symbols_list,qemu-plugins-ld64.symbols` is causing this to happen: only symbols listed in `qemu-plugins-ld64.symbols` would be kept in `qemu-system-*` binaries and all other symbols will be hidden.

Note that this is not caused by stripping (although I had to use custom strip command line on macOS to successfully compile qemu); the option `-exported_symbols_list` works by only exposing the provided symbols and treating all other symbols as `visibility=hidden`.

Replacing `--enable-plugins` to `--disable-plugins` in the above configure command line would "fix" it, although it means TCG plugins will not be supported.
Steps to reproduce:
1. Build QEMU on macOS with plugin support enabled
2. Try to use modules in shared library like qxl
Additional information:
Some examples:

```
$ qemu-system-x86_64 -device qxl
Failed to open module: dlopen(/usr/local/bin/../lib/qemu/ui-spice-core.dylib, 10): Symbol not found: __TRACE_QEMU_SPICE_ADD_MEMSLOT_DSTATE
  Referenced from: /usr/local/bin/../lib/qemu/ui-spice-core.dylib
  Expected in: flat namespace
 in /usr/local/bin/../lib/qemu/ui-spice-core.dylib
Failed to open module: dlopen(/usr/local/bin/../lib/qemu/hw-display-qxl.dylib, 2): Symbol not found: __TRACE_QXL_CLIENT_MONITORS_CONFIG_CAPPED_DSTATE
  Referenced from: /usr/local/bin/../lib/qemu/hw-display-qxl.dylib
  Expected in: flat namespace
 in /usr/local/bin/../lib/qemu/hw-display-qxl.dylib
qemu-system-x86_64: -device qxl: 'qxl' is not a valid device model name
```

```
$ qemu-system-x86_64 -spice port=5901
Failed to open module: dlopen(/usr/local/bin/../lib/qemu/ui-spice-core.dylib, 10): Symbol not found: __TRACE_QEMU_SPICE_ADD_MEMSLOT_DSTATE
  Referenced from: /usr/local/bin/../lib/qemu/ui-spice-core.dylib
  Expected in: flat namespace
 in /usr/local/bin/../lib/qemu/ui-spice-core.dylib
qemu-system-x86_64: -spice port=5901: spice support is disabled
```

After disabling plugin support I could run virtual machines locally through libvirt with full spice and qxl video support.