Allow configuration of the sector size advertised to the guest.

The default behavior is to infer the logical and physical sector sizes from
the block device backend. However older versions of Windows only work with
specific logical/physical combinations:

• Vista and Windows 7: 512/512
• Windows 7 SP1: 512/512 or 512/4096

For this reason allow the sector size to be specified using the following
block device option: sectorsize=logical[/physical]

Reported by: Leon Dang (ldang@nahannisys.com)
Reviewed by: grehan
MFC after: 2 weeks