bhyve/ioapic: only account for asserted line in level mode

After modifying a redirection entry only try to inject an interrupt if
the pin is in level mode, pins in edge mode shouldn't take into
account the line assert status as they are triggered by edge changes,
not the line status itself.

Reviewed by: grehan
Differential revision: https://reviews.freebsd.org/D28237

(cherry picked from commit d7d067698a38a3464a58eb34f68f63e529c45136)