ivshmem: use a single eventfd callback, get rid of CharDriver

ivshmem: use a single eventfd callback, get rid of CharDriver

Simplify the interrupt handling by having a single callback on irq&msi
cases. Remove usage of CharDriver, replace it with
qemu_set_fd_handler(). Use event_notifier_test_and_clear() to read the
eventfd.

Before this patch, ivshmem writes the first byte received to
s->intrstatus. But ivshmem_device_spec.txt says "The status register is
set to 1 when an interrupt occurs." Fortunately, the byte usually comes
from another ivshmem device, and those always write 1.

After this commit, follows the specification, set to 1 when an interrupt
occurs.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>