vmm ppt: Enable busmastering and BAR decoding while a device is assigned

Reviewed by: corvink, markj
Fixes: f44ff2aba2d6 bhyve: Treat the COMMAND register for PCI passthru devices as emulated
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D46245

(cherry picked from commit 776cd02b891ccd984963c9ec26f9748d213f0b9b)