pci-arbiter: Implement mapping for ROM files

[Joan Lledó]

Helllo Hurd,

I wrote a patch to allow mapping ROM files from the arbiter file system.

In order to achieve this, the arbiter first allocates and copies the entire 
device ROM in its space the first time it's accessed, and returns a proxy to 
that memory region when a client requests a ROM filemap.

This has the obvious drawback of each arbiter instance having to copy all 
devices ROMs into its space. It would be better to get the ROM physical address 
somehow and map it into the arbiter space the same way we do for BAR regions, 
but (AFAIK) libpciaccess doesn't provide a way for a client to get the ROM 
physical address, only the ROM size is available.