target/i386: disable VMX features if nested=0

target/i386: disable VMX features if nested=0

If kvm does not support VMX feature by nested=0, the kvm_vmx_basic
can't get the right value from MSR_IA32_VMX_BASIC register, which
make qemu coredump when qemu do KVM_SET_MSRS.

The coredump info:
error: failed to set MSR 0x480 to 0x0
kvm_put_msrs: Assertion `ret == cpu->kvm_msr_buf->nmsrs' failed.

Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20191206071111.12128-1-yang.zhong@intel.com>
Reported-by: Catherine Ho <catherine.hecx@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>