xen/x86: Don't try to set PCE bit in CR4

Since VPMU code emulates RDPMC instruction with RDMSR and because hypervisor
does not emulate it there is no reason to try setting CR4's PCE bit (and the
hypervisor will warn on seeing it set).

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>