KVM: x86: get_msr support for HV_X64_MSR_APIC_ASSIST_PAGE

KVM: x86: get_msr support for HV_X64_MSR_APIC_ASSIST_PAGE

"get" support for the HV_X64_MSR_APIC_ASSIST_PAGE msr was missing, even
though it is explicitly enumerated as something the vmm should save in
msrs_to_save and reported to userland via the KVM_GET_MSR_INDEX_LIST
ioctl.

Add "get" support for HV_X64_MSR_APIC_ASSIST_PAGE.  We simply return the
guest visible value of this register, which seems to be correct as a set
on the register is validated for us already.

Signed-off-by: Mike Waychison <mikew@google.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>