libxl: refactor DISK_BACKEND_PHY handling in libxl_device_disk_add

libxl: refactor DISK_BACKEND_PHY handling in libxl_device_disk_add

A step on the path to sharing this code with the tail-end of the
DISK_BACKEND_TAP case.

I made the result of libxl__blktap_devpath non-const to achieve
this. The existing caller calls libxl__strdup on the result but since
the function is an internal one and the result is already garbage
collected I think this is unnecessary and we can just use the
non-const result directly.

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