Revert "hw/i386/pc_sysfw: Inline pc_system_flash_create() and remove it"

Revert "hw/i386/pc_sysfw: Inline pc_system_flash_create() and remove it"

Commit 6f6ad2b24582 "hw/i386/pc: Confine system flash handling to pc_sysfw"
causes a regression when specifying the property `-M pflash0` in the PCI PC
machines:
  qemu-system-x86_64: Property 'pc-q35-9.0-machine.pflash0' not found
In order to revert the commit, the commit below must be reverted first.

This reverts commit cb05cc16029bb0a61ac5279ab7b3b90dcf2aa69f.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20240226215909.30884-2-shentey@gmail.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>