pygrub: cope better with big files in the guest.

pygrub: cope better with big files in the guest.

Only read the first megabyte of a configuration file (grub etc.) and read the
kernel and ramdisk files from the guest in one megabyte pieces so pygrub
doesn't use a lot of memory if the files are large.  With --not-really option
check that the chosen kernel and ramdisk files exist.  If there are problems
writing the copy of the kernel or ramdisk, delete the copied files and exit in
case they have filled the filesystem.

Signed-off-by: Michael Young <m.a.young@durham.ac.uk>
Acked-by: Matt Wilson <msw@amazon.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>