vswitch: Do not use "xe" on XenServer, to avoid deadlock.

vswitch: Do not use "xe" on XenServer, to avoid deadlock.

Commit 4ab581b949, "vswitch: Fix duplicated DPIDs observed under
XenServer," changed vswitchd to use the "xe" program to obtain the host
UUID and network name under XenServer.  Unfortunately, on XenServer pool
slaves this causes a deadlock, because on such machines "xe" always
attempts to contact the master to perform database queries.  Since vswitchd
is the entity that must set up the flow that "xe" initiates as part of
that connection, this will deadlock and fail.

Instead, get the host UUID from /etc/xensource-inventory and hash it with
the bridge name.  Discussion with the NOX folks indicates that this should
provide sufficient stability and uniqueness.