'xe sr-create' now allows device-config parameter values to be uploaded from files.

We can now say:

  xe sr-create type=libvirt name-label=ceph device-config:xml-filename=ceph-sr.xml

and the local file 'ceph-sr.xml' will be uploaded as the value for the device-config
key 'xml'.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>