vmm: Enable unprivileged bhyve

- Add the vmm group.
- Let /dev/vmmctl belong to the vmm group by default, and give group
  write permissions.
- When creating a VM's device files, make them owned by the creating
  process' effective UID.

Reviewed by:	bnovkov
MFC after:	2 months
Sponsored by:	The FreeBSD Foundation
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D54741

etc/group

@@ -34,5 +34,6 @@ ntpd:*:123:
 _ypldap:*:160:
 hast:*:845:
 tests:*:977:
+vmm:*:978:
 nogroup:*:65533:
 nobody:*:65534:

sys/dev/vmm/vmm_dev.c

@@ -1033,8 +1033,8 @@ vmmdev_create(const char *name, uint32_t flags, struct ucred *cred)
 	make_dev_args_init(&mda);
 	mda.mda_devsw = &vmmdevsw;
 	mda.mda_cr = sc->ucred;
-	mda.mda_uid = UID_ROOT;
-	mda.mda_gid = GID_WHEEL;
+	mda.mda_uid = cred->cr_uid;
+	mda.mda_gid = GID_VMM;
 	mda.mda_mode = 0600;
 	mda.mda_si_drv1 = sc;
 	mda.mda_flags = MAKEDEV_CHECKNAME | MAKEDEV_WAITOK;
@@ -1201,7 +1201,7 @@ vmmdev_init(void)
 
 	sx_xlock(&vmmdev_mtx);
 	error = make_dev_p(MAKEDEV_CHECKNAME, &vmmctl_cdev, &vmmctlsw, NULL,
-	    UID_ROOT, GID_WHEEL, 0600, "vmmctl");
+	    UID_ROOT, GID_VMM, 0660, "vmmctl");
 	if (error == 0) {
 		pr_allow_vmm_flag = prison_add_allow(NULL, "vmm", NULL,
 		    "Allow use of vmm in a jail");
@@ -1357,8 +1357,8 @@ devmem_create_cdev(struct vmmdev_softc *sc, int segid, char *devname)
 	make_dev_args_init(&mda);
 	mda.mda_devsw = &devmemsw;
 	mda.mda_cr = sc->ucred;
-	mda.mda_uid = UID_ROOT;
-	mda.mda_gid = GID_WHEEL;
+	mda.mda_uid = sc->ucred->cr_uid;
+	mda.mda_gid = GID_VMM;
 	mda.mda_mode = 0600;
 	mda.mda_si_drv1 = dsc;
 	mda.mda_flags = MAKEDEV_CHECKNAME | MAKEDEV_WAITOK;

sys/sys/conf.h

@@ -160,6 +160,7 @@ typedef int dumper_hdr_t(struct dumperinfo *di, struct kerneldumpheader *kdh);
 #define		GID_ID_PRIO	48
 #define		GID_DIALER	68
 #define		GID_U2F		116
+#define		GID_VMM		978
 #define		GID_NOGROUP	65533
 #define		GID_NOBODY	65534