vmm/svm: iopm_bitmap and msr_bitmap must be contiguous in physical memory

To achieve that the whole svm_softc is allocated with contigmalloc now.
It would be more effient to de-embed those arrays and allocate only them
with contigmalloc.

Previously, if malloc(9) used non-contiguous pages for the arrays, then
random bits in physical pages next to the first page would be used to
determine permissions for I/O port and MSR accesses. That could result
in a guest dangerously modifying the host hardware configuration.

One example is that sometimes NMI watchdog driver in a Linux guest
would be able to configure a performance counter on a host system.
The counter would generate an interrupt and if hwpmc(4) driver is loaded
on the host, then the interrupt would be delivered as an NMI.

Discussed with: jhb
Reviewed by: grehan
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D8321