Nested VMX: Clear bit 31 of IA32_VMX_BASIC MSR

Nested VMX: Clear bit 31 of IA32_VMX_BASIC MSR

The bit 31 of revision_id will set to 1 if vmcs shadowing enabled. And
according intel SDM, the bit 31 of IA32_VMX_BASIC MSR is always 0. So we
cannot set low 32 bit of IA32_VMX_BASIC to revision_id directly. Must clear
the bit 31 to 0.

Signed-off-by: Yang Zhang <yang.z.zhang@Intel.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>