hw/char/serial.h: Extract serial-isa.h

hw/char/serial.h: Extract serial-isa.h

The includes where updated based on compile errors. Now, the inclusion of the
header roughly matches Kconfig dependencies:

  # grep -r -e "select SERIAL_ISA"
  hw/ppc/Kconfig:    select SERIAL_ISA
  hw/isa/Kconfig:    select SERIAL_ISA
  hw/sparc64/Kconfig:    select SERIAL_ISA
  hw/i386/Kconfig:    select SERIAL_ISA
  hw/i386/Kconfig:    select SERIAL_ISA # for serial_hds_isa_init()

Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Link: https://lore.kernel.org/r/20240905073832.16222-3-shentey@gmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>