virtio_pci_legacy: Allow memory space for configuration

For guests running under some kind of VMMs, configuration structure is
available in memory space but not I/O space.

Reported by: Yuan Rui <number201724@me.com>
MFC after: 2 weeks
Reviewed by: rpokala, bryanv, jhb
Approved by: philip (mentor)
Differential Revision: https://reviews.freebsd.org/D28818