x86/vvmx: Fix the use of RDTSCP when it is intercepted at L0

x86/vvmx: Fix the use of RDTSCP when it is intercepted at L0

Linux has started using RDTSCP as of v5.1.  This has highlighted a bug in Xen,
where virtual vmexit simply gives up.

  (XEN) d1v1 Unhandled nested vmexit: reason 51
  (XEN) domain_crash called from vvmx.c:2671
  (XEN) Domain 1 (vcpu#1) crashed on cpu#2:

Handle RDTSCP in the virtual vmexit hander in the same was as RDTSC
intercepts.

Reported-by: Sarah Newman <srn@prgmr.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Chris Brannon <cmb@prgmr.com>
Reviewed-by: Wei Liu <wl@xen.org>
Release-acked-by: Juergen Gross <jgross@suse.com>