kvmtool: ARM: allow level interrupts in device tree

Currently we describe every interrupt for each device in the FDT
as being edge triggered.
Add a parameter to the irq property generation to allow devices to
specify their interrupts as level triggered if needed.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>