x86: disable VPMU for PVH guests

x86: disable VPMU for PVH guests

Currently when VPMU is enabled on a system both HVM and PVH VPCUs will
initialize their VPMUs, including setting up vpmu_ops. As result even
though VPMU will not work for PVH guests (APIC is not supported there),
the guest may decide to perform a write to a PMU MSR. This will cause a
call to is_vlapic_lvtpc_enabled() which will crash the hypervisor, e.g.:

 (XEN) Xen call trace:
 (XEN)    [<ffff82d0801ca06f>] is_vlapic_lvtpc_enabled+0x13/0x22
 (XEN)    [<ffff82d0801e2a15>] core2_vpmu_do_wrmsr+0x415/0x589
 (XEN)    [<ffff82d0801cedaa>] vpmu_do_wrmsr+0x2a/0x33
 (XEN)    [<ffff82d0801dd648>] vmx_msr_write_intercept+0x268/0x557
 (XEN)    [<ffff82d0801bcd2e>] hvm_msr_write_intercept+0x36c/0x39b
 (XEN)    [<ffff82d0801e0a0e>] vmx_vmexit_handler+0x1082/0x185b
 (XEN)    [<ffff82d0801e74c1>] vmx_asm_vmexit_handler+0x41/0xc0

If we prevent VPMU from being initialized on PVH guests we will avoid
those accesses.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Release-Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>