libxl: correctly parse storage devices on driver domains

libxl: correctly parse storage devices on driver domains

Don't try to check physical devices if they belong to a domain
different than the one where the toolstack is running. This prevents
the following error when trying to use storage driver domains:

libxl: debug: libxl_create.c:1246:do_domain_create: ao 0x1819240: create: how=(nil) callback=(nil) poller=0x1818fa0
libxl: debug: libxl_device.c:235:libxl__device_disk_set_backend: Disk vdev=xvda spec.backend=phy
libxl: debug: libxl_device.c:175:disk_try_backend: Disk vdev=xvda, backend phy unsuitable as phys path not a block device
libxl: error: libxl_device.c:278:libxl__device_disk_set_backend: no suitable backend for disk xvda

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>