sys_util: allow clearing given pending signal
sys_util: allow clearing given pending signal

We are planning on using KVM_SET_SIGNAL_MASK and have the signal that we
use to kick VCPU permanently masked to close the race around handling
pause requests, so we need a way to clear pending interrupts, otherwise
VM will never run again.

TEST=cargo test --features plugin; cargo test -p kvm; ./build_test
BUG=chromium:800626

Change-Id: I2dfe6fcb129e4b8156f6a7ff842e171661c56440
Signed-off-by: Dmitry Torokhov <10a8c465cefc9bdd6c925e26964d23c90f1141cc@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/930462
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm