target/i386: Added VGIF feature

VGIF allows STGI and CLGI to execute in guest mode and control virtual
interrupts in guest mode.
When the VGIF feature is enabled then:
 * executing STGI in the guest sets bit 9 of the VMCB offset 60h.
 * executing CLGI in the guest clears bit 9 of the VMCB offset 60h.

Signed-off-by: Lara Lazier <laramglazier@gmail.com>
Message-Id: <20210730070742.9674-1-laramglazier@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>