x86: Implement vcpu_[de]activate_vmm() for AMD-V

x86: Implement vcpu_[de]activate_vmm() for AMD-V

Compared to VMX, vcpu_activate_vmm() and vcpu_deativate_vmm() for AMD-V
systems have several notable differences.

First, additional MSRs (part of VMCB but not VMCS) need to be set and
restored. Then, host state area is opaque in AMD-V, so vcpu_activate_vmm()
remembers host stack pointer and essentially begins a VMRUN/#VMEXIT loop
implemented in svm-vmexit.S. Third, as RAX register is part of VMCB, it is
copied to guest_regs on each VM exit and written back before VM entry.

As Jailhouse runs with GIF set, it needs to be cleared on VMM deactivation.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>