vmm: Restore the ability to create VMs as root in a jail

The new PRIV_VMM_CREATE and DESTROY permissions should be allowed by
jails, so need to be added to the list in prison_priv_check(). Then,
modify vmmdev_create() to verify that the jail was created with the
allow.vmm flag. This is already verified when opening /dev/vmmctl, but
checking again doesn't hurt and ensures that one can't pass the
allow.vmm policy by passing a vmmctl fd along a unix domain socket from
outside the jail.

Rename vmm_priv_check() to vmm_jail_priv_check() to make the function's
purpose more clear.

Reported by: novel
Reviewed by: bnovkov
Fixes: d4c05edd410e ("vmm: Add privilege checks to vmmctl operations")
Differential Revision: https://reviews.freebsd.org/D56119