KVM: ia64: enable external interrupt in vmm

Currently, the interrupt enable bit is cleared when in
the vmm.  This patch sets the bit and the external interrupts can
be dealt with when in the vmm.  This improves the I/O performance.

Signed-off-by: Yang Zhang <yang.zhang@intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>