CA-119317: Consider VDI.sm_config:backend-kind
CA-119317: Consider VDI.sm_config:backend-kind

Allow a way of an SM driver to configure the VBD backend to be used for this
VDI. This is done by setting the backend-kind key in the sm_config field.

This value will be used but will still be overridden if a value appears in
other-config (user-configurable).

Signed-off-by: Si Beaumont <a0992b7f1f450b4c7716d34b81a2d6f0877ae11a@citrix.com>

cheng--zhang/xen-api,euanh/xen-api,djs55/xen-api,vasilenkomike/xen-api,jjd27/xen-api,cheng--zhang/xen-api,jjd27/xen-api,anoobs/xen-api,salvocambria/xen-api,djs55/xen-api,koushikcgit/xen-api,agimofcarmen/xen-api,rafalmiel/xen-api,robertbreker/xen-api,vasilenkomike/xen-api,rafalmiel/xen-api,euanh/xen-api,guard163/xen-api,cheng-z/xen-api,djs55/xen-api,srowe/xen-api,cheng--zhang/xen-api,guard163/xen-api,guard163/xen-api,jjd27/xen-api,anoobs/xen-api,robertbreker/xen-api,thomassa/xen-api,anoobs/xen-api,rafalmiel/xen-api,rafalmiel/xen-api,Frezzle/xen-api,agimofcarmen/xen-api,cheng-z/xen-api,euanh/xen-api,srowe/xen-api,huizh/xen-api,koushikcgit/xen-api,Frezzle/xen-api,srowe/xen-api,vasilenkomike/xen-api,huizh/xen-api,djs55/xen-api,thomassa/xen-api,thomassa/xen-api,salvocambria/xen-api,thomassa/xen-api,simonjbeaumont/xen-api,rafalmiel/xen-api,vasilenkomike/xen-api,koushikcgit/xen-api,agimofcarmen/xen-api,robertbreker/xen-api,srowe/xen-api,Frezzle/xen-api,euanh/xen-api,djs55/xen-api,salvocambria/xen-api,euanh/xen-api,huizh/xen-api,simonjbeaumont/xen-api,jjd27/xen-api,euanh/xen-api,cheng--zhang/xen-api,cheng-z/xen-api,simonjbeaumont/xen-api,simonjbeaumont/xen-api,agimofcarmen/xen-api,huizh/xen-api,cheng-z/xen-api,vasilenkomike/xen-api,koushikcgit/xen-api,cheng--zhang/xen-api,anoobs/xen-api,robertbreker/xen-api,cheng--zhang/xen-api,huizh/xen-api,srowe/xen-api,cheng-z/xen-api,guard163/xen-api,salvocambria/xen-api,Frezzle/xen-api,anoobs/xen-api,koushikcgit/xen-api,thomassa/xen-api,vasilenkomike/xen-api,cheng-z/xen-api,cheng-z/xen-api,anoobs/xen-api,guard163/xen-api,thomassa/xen-api,agimofcarmen/xen-api,djs55/xen-api,robertbreker/xen-api,guard163/xen-api,salvocambria/xen-api,simonjbeaumont/xen-api,robertbreker/xen-api,salvocambria/xen-api,rafalmiel/xen-api,Frezzle/xen-api,agimofcarmen/xen-api,jjd27/xen-api,huizh/xen-api,simonjbeaumont/xen-api,jjd27/xen-api,cheng--zhang/xen-api,koushikcgit/xen-api