x86/hvm: Constify the read side of vlapic handling

x86/hvm: Constify the read side of vlapic handling

This is in preparation to make hvm_x2apic_msr_read() take a const vcpu
pointer.  One modification is to alter vlapic_get_tmcct() to not use current.

This in turn needs an alteration to hvm_get_guest_time_fixed(), which is safe
because the only mutable action it makes is to take the domain plt lock.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>