[CORE] vmm_devemu: Minor fix in vmm_devemu_deinit_context()

The vmm_devemu_deinit_context() function can be called when guest
is paritially initialized and fails at VCPU creation time. In this
case, vmm_devemu_deinit_context() will be called before function
vmm_devemu_init_context() is called hence will crash when trying
to free context pointed by "guest->aspace.devemu_priv".

This patch fixes above described issue in vmm_devemu_deinit_context().

Signed-off-by: Anup Patel <anup@brainfault.org>