x86/boot: re-arrange how/when we do disk I/O

x86/boot: re-arrange how/when we do disk I/O

We place the trampoline no lower than at 256k, so we have ample space
to read the MBRs of BIOS disks into an aligned buffer right below the
trampoline (not doing so has been found to be a problem on a buggy BIOS
coming with a Skull Canyon NUC). To facilitate that move MBR reading
past EDD info retrieval.

Also add a wrap check to the EDD info retrieval loop, to match that in
the MBR reading one.

Reported-by: Paul Durrant <Paul.Durrant@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Paul Durrant <Paul.Durrant@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>