SR.create and SR.attach both take libvirt storage pool XML

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Remember which libvirt storage pools we're using; reconnect on service restart.

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

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