bhyve: Consolidate NVMe CQ update

Consolidate the code which writes Completion Queue entries and updates
the CQ doorbell value. While in the neighborhood, convert the "toggle CQ
phase bit" code to use an XOR operation instead of an "if/else" branch.

Tested by: Jason Tubnor
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24882