bhyve: Move the gvt-d driver to amd64/

It is amd64-only.  No functional change intended.

Reviewed by:	corvink, jhb
MFC after:	1 week
Sponsored by:	Innovate UK
Differential Revision:	https://reviews.freebsd.org/D40736

usr.sbin/bhyve/Makefile

@@ -41,7 +41,6 @@ SRCS=	\
 	pci_e82545.c		\
 	pci_emul.c		\
 	pci_fbuf.c		\
-	pci_gvt-d.c		\
 	pci_hda.c		\
 	pci_hostbridge.c	\
 	pci_irq.c		\

usr.sbin/bhyve/amd64/Makefile.inc

@@ -4,6 +4,7 @@ SRCS+=	\
 	fwctl.c		\
 	kernemu_dev.c	\
 	mptbl.c		\
+	pci_gvt-d.c	\
 	pm.c		\
 	post.c		\
 	ps2kbd.c	\