xen/arm: Call context_saved() with interrupts enabled during context switch

xen/arm: Call context_saved() with interrupts enabled during context switch

This is a requirement of the scheduler interface, violating this
causes for example with the RT scheduler:

(XEN) Assertion 'local_irq_is_enabled()' failed at spinlock.c:137
(XEN) ----[ Xen-4.5.0  arm32  debug=y  Not tainted ]----
(XEN) CPU:    0
(XEN) PC:     0022a074 _spin_lock_irq+0x18/0x94

(XEN) Xen call trace:
(XEN)    [<0022a074>] _spin_lock_irq+0x18/0x94 (PC)
(XEN)    [<002256b8>] rt_context_saved+0x3c/0x144 (LR)
(XEN)    [<002256b8>] rt_context_saved+0x3c/0x144
(XEN)    [<00228ed8>] context_saved+0x4c/0x80
(XEN)    [<002524b4>] schedule_tail+0x148/0x2f0
(XEN)    [<00252668>] continue_new_vcpu+0xc/0x70
(XEN)    [<00252a18>] context_switch+0x74/0x7c

Signed-off-by: denys drozdov <denys.drozdov@globallogic.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>