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>