Allow a 4-byte write to PCI config space to overlap

Allow a 4-byte write to PCI config space to overlap
the 2 read-only bytes at the start of a PCI capability.
This is the sequence that OpenBSD uses when enabling
MSI interrupts, and works fine on real h/w.

In bhyve, convert the 4 byte write to a 2-byte write to
the r/w area past the first 2 r/o bytes of a capability.

Reviewed by:	neel
Approved by:	re@ (blanket)