sparse_dd: default to respecting the user's choice of encryption mode

Forcibly enabling or disabling system-wide is only intended as a
bug workaround elsewhere.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
sparse_dd: never rewrite an http:// or https:// URL as a file:// URL

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
sparse_dd: log the intended destination after the URL rewrite step

Otherwise the logs will say one thing and we will do another.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
sparse_dd: update config file to show default configuration

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add the notion of a 'vhd search path' for finding parents with relative paths

XenServer makes parent locators with relative paths which are
interpreted (by libvhd) as being relative to "/dev/mapper" (Linux
device mapper)

The CLI vhd-tool now takes an optional argument
  --path /foo:/bar

... while sparse_dd (the XenServer) tool has hardcoded "/dev/mapper:."

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #6 from djs55/bugfixes

sparse_dd bugfixes