Revert "memory: Drop FlatRange.romd_mode"

Revert "memory: Drop FlatRange.romd_mode"

This reverts commit 5b5660adf1fdb61db14ec681b10463b8cba633f1,
as it breaks the UEFI guest firmware (known as ArmVirtPkg or AAVMF)
running in the "virt" machine type of "qemu-system-aarch64":

Contrary to the commit message, (a->mr == b->mr) does *not* imply
that (a->romd_mode == b->romd_mode): the pflash device model calls
memory_region_rom_device_set_romd() -- for switching between the above
modes --, and that function changes mr->romd_mode but the current
AddressSpaceDispatch's FlatRange keeps the old value.  Therefore
region_del/region_add are not called on the KVM MemoryListener.

Reported-by: Drew Jones <drjones@redhat.com>
Tested-by: Drew Jones <drjones@redhat.com>
Analyzed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>