libxlu: correctly parse disk "backendtype" field

libxlu: correctly parse disk "backendtype" field

Currently it tries to parse the value from the full "backendtype=FOO" string
but really it needs to parse from the equals.

Before:
# xl -N block-attach d32-1 backendtype=phy,vdev=xvdb,access=w,target=/dev/VG/debian-x86_32-1b
command line: config parsing error in disk specification: unknown value for backendtype: near `backendtype=phy' in `backendtype=phy,vdev=xvdb,access=w,target=/dev/VG/debian-x86_32-1b'

After [in new syntax, not yet in this tree -iwj]:
# xl -N block-attach d32-1 backendtype=phy,vdev=xvdb,access=w,target=/dev/VG/debian-x86_32-1b
disk: {
    "backend_domid": 0,
    "pdev_path": "/dev/VG/debian-x86_32-1b",
    "vdev": "xvdb",
    "backend": "phy",
    "format": "raw",
    "script": null,
    "removable": 0,
    "readwrite": 1,
    "is_cdrom": 0
}

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>