KVM/VMX: Clear spec_ctrl status when resetting vcpu

KVM/VMX: Clear spec_ctrl status when resetting vcpu

vmx->spec_ctrl was not set to 0 in vmx_vcpu_reset, which could result in
IBRS getting stuck on all the time, even with 'spectre_v2=off' set. This
was most notable when rebooting from an older kernel into a newer
retpoline-enabled kernel resulted in up to 80% CPU performance drop.

OraBug: 27774415

Suggested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Patrick Colp <patrick.colp@oracle.com>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>