ioeventfd: Fix removal of ioeventfd

Fix three bugs that prevent removal of ioeventfds in KVM. Store the
flags in the right structure, check the datamatch parameter, and pass
the fd to KVM.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>