pvh: Fix regression due to assumption that HVM paths MUST use io-backend device

pvh: Fix regression due to assumption that HVM paths MUST use io-backend device

The commit 09bb434748af9bfe3f7fca4b6eef721a7d5042a4
"Nested VMX: prohibit virtual vmentry/vmexit during IO emulation"
assumes that the HVM paths are only taken by HVM guests. With the PVH
enabled that is no longer the case - which means that we do not have
to have the IO-backend device (QEMU) enabled.

As such, that patch can crash the hypervisor:

Xen call trace:
    [<ffff82d0801ddd9a>] nvmx_switch_guest+0x4d/0x903
    [<ffff82d0801de95b>] vmx_asm_vmexit_handler+0x4b/0xc0

Pagetable walk from 000000000000001e:
  L4[0x000] = 0000000000000000 ffffffffffffffff

****************************************
Panic on CPU 7:
FATAL PAGE FAULT
[error_code=0000]
Faulting linear address: 000000000000001e
****************************************

as we do not have an io based backend. In the case that the
PVH guest does run an HVM guest inside it - we need to do
further work to suport this - and for now the check will
bail us out.

We also fix spelling mistakes and the sentence structure.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Release-acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: "Zhang, Yang Z" <yang.z.zhang@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>