XenAPI: SR location should default to location stored in PBD

This value must be read from the PBD as it may change for different
installations.  The existing default is preserved as a fallback
for the cases where a PBD path is not set.

Implements blueprint xenserver-core

Change-Id: I055ebe8d7d79f238b0b9565277acac945bbe12e1
Merge "XenAPI: SR location should default to location stored in PBD"