vpc: Add support for Fixed Disk type

vpc: Add support for Fixed Disk type

The Virtual Hard Disk Image Format Specification allows for three
types of hard disk formats, Fixed, Dynamic, and Differencing.  Qemu
currently only supports Dynamic disks.  This patch adds support for
the Fixed Disk format.

Usage:
    Example 1: qemu-img create -f vpc -o type=fixed <filename> [size]
    Example 2: qemu-img convert -O vpc -o type=fixed <input filename> <output filename>

While it is also allowed to specify '-o type=dynamic', the default disk type
remains Dynamic and is what is used when the type is left unspecified.

Signed-off-by: Charles Arnold <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>