pvh: restrict tsc_mode to NEVER_EMULATE for now

pvh: restrict tsc_mode to NEVER_EMULATE for now

The reason given for this restriction in the first place, given in one
of the comments checking for PVH requirements, had to do with
additional infrastructure required to allow PV RDTSC emulation for PVH
guests.

Since we don't use the PV emulation path at all anymore, we may be
able to remove this restriction.

Experiments show that pvh will boot without apparent issues in
"default", "native", and "native_paravirt" mode, but not in
"always_emulate" mode.  We'll leave this restriction in until
we can sort out what's going on.

Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Eddie Dong <eddie.dong@intel.com>