CP-4147: Change the USB emulation platform flag semantics

Instead of having a single flag, "nousb", which turns off all USB
emulation, we now have two flags:

  - "usb", which controls USB emulation
  - "usb_tablet", which controls USB emulation, as long as USB emulation
    is turned on

xapi defaults to both keys being true, whereas xenopsd defaults to both
keys being false.

Signed-off-by: John Else <john.else@citrix.com>
Signed-off-by: David Scott <dave.scott@eu.citrix.com>