According to the documentation in memory.h a ROM memory region will be
backed by RAM for reads, but is supposed to go through a callback for
writes. Currently we were not checking for the existence of the rom_device
flag when determining if we could perform a direct write or not.
To correct that add a check to memory_region_is_direct so that if the
memory region has the rom_device flag set we will return false for all
checks where is_write is set.
Backports commit d489ae4ac57ebe14bde8384556cbac237ead988d from qemu