Add helper struct to manage device interrupt mode

A device may support multiple interrupt modes. For example, a PCI
device may support legacy, PCI MSI and PCI MSIx interrupts. So add
struct DeviceInterruptManager to manage the device interupt working
mode. This interrupt manager helps a device backend driver to manage
its interrupts and provides interfaces to switch interrupt working
modes.

Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>

Author: jiangliu

Cargo.toml

@@ -13,10 +13,15 @@ kvm-ioctls = { version = "~0", optional = true }
 vfio-ioctls = { git = "https://github.com/cloud-hypervisor/vfio-ioctls.git", branch = "dragonball", optional = true }
 vmm-sys-util = "~0"
 
+[dev-dependencies]
+byteorder = ">=1.2.1"
+
 [features]
+default = ["legacy-irq", "msi-irq"]
 legacy-irq = []
 msi-irq = []
 vfio-msi-irq = []
+
 kvm-irq = ["kvm-ioctls", "kvm-bindings"]
 kvm-msi-generic = ["msi-irq", "kvm-irq"]
 kvm-legacy-irq = ["legacy-irq", "kvm-irq"]