KVM: SVM: Move RESET emulation to svm_vcpu_reset()

Move RESET emulation for SVM vCPUs to svm_vcpu_reset(), and drop an extra
init_vmcb() from svm_create_vcpu() in the process.  Hopefully KVM will
someday expose a dedicated RESET ioctl(), and in the meantime separating
"create" from "RESET" is a nice cleanup.

Keep the call to svm_switch_vmcb() so that misuse of svm->vmcb at worst
breaks the guest, e.g. premature accesses doesn't cause a NULL pointer
dereference.

Cc: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210921000303.400537-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>