Nested VMX: prohibit virtual vmentry/vmexit during IO emulation

Nested VMX: prohibit virtual vmentry/vmexit during IO emulation

Sometimes, L0 needs to decode L2's instruction to handle IO access directly.
And L0 may get X86EMUL_RETRY when handling this IO request. At same time, if
there is a virtual vmexit pending (for example, an interrupt pending to inject
to L1) and hypervisor will switch the VCPU context from L2 to L1. Now we
already are in L1's context, but since we got a X86EMUL_RETRY just now and
this means hypervisor will retry to handle the IO request later and
unfortunately, the retry will happen in L1's context. And it will cause the
problem. The fixing is that if there is a pending IO request, no virtual
vmexit/vmentry is allowed.

Signed-off-by: Yang Zhang <yang.z.zhang@Intel.com>
Acked-by: Jun Nakajima <jun.nakajima@intel.com>