pygrub: fix read sizes on FreeBSD

pygrub: fix read sizes on FreeBSD

FreeBSD only allows reading multiples of sector size from raw disk devices
(character devices). This fix should only alter the behaviour of pygrub on
FreeBSD, the other supported OSes will continue using the same size.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>