hyperv: Detect Nested virtualization support for SVM

Previously, to detect nested virtualization enlightenment support,
we were using HV_X64_ENLIGHTENED_VMCS_RECOMMENDED feature bit of
HYPERV_CPUID_ENLIGHTMENT_INFO.EAX CPUID as docuemented in TLFS:
 "Bit 14: Recommend a nested hypervisor using the enlightened VMCS
  interface. Also indicates that additional nested enlightenments
  may be available (see leaf 0x4000000A)".

Enlightened VMCS, however, is an Intel only feature so the above
detection method doesn't work for AMD. So, use the
HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS.EAX CPUID information ("The
maximum input value for hypervisor CPUID information.") and this
works for both AMD and Intel.

Signed-off-by: Vineeth Pillai <viremana@linux.microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Message-Id: <43b25ff21cd2d9a51582033c9bdd895afefac056.1622730232.git.viremana@linux.microsoft.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>