KVM: selftests: Split off load_evmcs() from load_vmcs()

In preparation to putting Hyper-V specific test pages to a dedicated
struct, move eVMCS load logic from load_vmcs(). Tests call load_vmcs()
directly and the only one which needs 'enlightened' version is
evmcs_test so there's not much gain in having this merged.

Temporary pass both GPA and HVA to load_evmcs().

Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20221101145426.251680-42-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>