qga: Allow building of the guest agent without system emulators or tools

qga: Allow building of the guest agent without system emulators or tools

If configuring with "--disable-system --disable-user --enable-guest-agent"
the linking currently fails with:

qga/qemu-ga.p/commands.c.o: In function `qmp_command_info':
build/../../home/thuth/devel/qemu/qga/commands.c:70: undefined reference to `qmp_command_name'
build/../../home/thuth/devel/qemu/qga/commands.c:71: undefined reference to `qmp_command_is_enabled'
build/../../home/thuth/devel/qemu/qga/commands.c:72: undefined reference to `qmp_has_success_response'
qga/qemu-ga.p/commands.c.o: In function `qmp_guest_info':
build/../../home/thuth/devel/qemu/qga/commands.c:82: undefined reference to `qmp_for_each_command'
qga/qemu-ga.p/commands.c.o: In function `qmp_guest_exec':
build/../../home/thuth/devel/qemu/qga/commands.c:410: undefined reference to `qbase64_decode'
qga/qemu-ga.p/channel-posix.c.o: In function `ga_channel_open':
build/../../home/thuth/devel/qemu/qga/channel-posix.c:214: undefined reference to `unix_listen'
build/../../home/thuth/devel/qemu/qga/channel-posix.c:228: undefined reference to `socket_parse'
build/../../home/thuth/devel/qemu/qga/channel-posix.c:234: undefined reference to `socket_listen'
qga/qemu-ga.p/commands-posix.c.o: In function `qmp_guest_file_write':
build/../../home/thuth/devel/qemu/qga/commands-posix.c:527: undefined reference to `qbase64_decode'

Let's make sure that we also compile and link the required files if
the system emulators have not been enabled.

Message-Id: <20221110083626.31899-1-thuth@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>