hw/i386/Kconfig: Let the MicroVM machine select the SERIAL_ISA config

hw/i386/Kconfig: Let the MicroVM machine select the SERIAL_ISA config

When configured with --without-default-devices, the build fails:

     LINK    x86_64-softmmu/qemu-system-x86_64
  /usr/bin/ld: hw/i386/microvm.o: in function `microvm_devices_init':
  hw/i386/microvm.c:157: undefined reference to `serial_hds_isa_init'
  collect2: error: ld returned 1 exit status
  make[1]: *** [Makefile:206: qemu-system-x86_64] Error 1
  make: *** [Makefile:483: x86_64-softmmu/all] Error 2

While the MicroVM machine only uses the ISA serial port when the
MICROVM_MACHINE_ISA_SERIAL property is set, it has to be linked
with it. Replace the 'imply' Kconfig rule by a 'select'.

Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20191231183216[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>