eal: add command line option to select vfio interrupt type

Unlike igb_uio, VFIO interrupt type is not set by kernel module
parameters but is set up via ioctl() calls at runtime. This warrants
a new EAL command-line parameter. It will have no effect if VFIO is
not compiled, but will set VFIO interrupt type to either "legacy", "msi"
or "msix" if VFIO support is compiled. Note that VFIO initialization
will fail if the interrupt type selected is not supported by the system.

If the interrupt type parameter wasn't specified, VFIO will try all
interrupt types (starting with MSI-X).

In unit tests, we don't know if VFIO is compiled (eal_vfio.h header is
internal to Linuxapp EAL), so we check this flag regardless.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: Waterman Cao <waterman.cao@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>