CA-107700 - Default to turning USB emulation on in xenopsd

xapi already defaults to turning this on, but during rolling-pool
upgrade the platform data gets passed directly from the sending to the
receiving xenopsd without xapi adding its usual defaults.

Therefore xenopsd also needs to default to turning this on, to match
previous releases.

Signed-off-by: John Else <john.else@citrix.com>