vmm: don't lock a mtx in the icr_low write handler

x2apic accesses are handled by a wrmsr exit. This handler is called in a
critical section. So, we can't lock a mtx in the icr_low handler.

Reported by: kp, pho
Tested by: kp, pho
Approved by: manu (mentor)
Fixes: c0f35dbf19c3c8825bd2b321d8efd582807d1940 vmm: Use a cpuset_t for vCPUs waiting for STARTUP IPIs.
MFC after: 1 week
MFC with: c0f35dbf19c3c8825bd2b321d8efd582807d1940
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D37452

(cherry picked from commit 7c326ab5bb9aced8dcbc2465ac1c9ff8df2ba46b)