x86 hvm: suspend platform timer emulation while its IRQ is masked

x86 hvm: suspend platform timer emulation while its IRQ is masked

This patch gets rid of a timer which IRQ is masked from vcpu's timer
list. It reduces the overhead of VM EXIT and context switch of vm.

Also fixes a potential bug.
(1) VCPU#0: mask the IRQ of a timer. (ex. vioapic.redir[2].mask=1)
(2) VCPU#1: pt_timer_fn() is invoked by expiration of the timer.
(3) VCPU#1: pt_update_irq() is called but does nothing by
pt_irq_masked()==1.
(4) VCPU#1: sleep by halt.
(5) VCPU#0: unmask the IRQ of the timer.
After that, no one wakes up the VCPU#1.

IRQ of ISA is masked by:
 - PIC's IMR
 - IOAPIC's redir[0]
 - IOAPIC's redir[N].mask
 - LAPIC's LVT0
 - LAPIC enabled/disabled

IRQ of LAPIC timer is masked by:
 - LAPIC's LVTT
 - LAPIC disabled

When above stuffs are changed, the corresponding vcpu is kicked and
suspended timer emulation is resumed.

In addition, a small bug fix in pt_adjust_global_vcpu_target().

Signed-off-by: Kouya Shimura <kouya@jp.fujitsu.com>