bhyve(8): Add bootrom allocation abstraction

To allow more general use of the bootrom region, separate initialization from
allocation, and allocation from loading a file.

The bootrom segment is the high 16MB of the low 4GB region.

Each allocation in the segment creates a new mapping with specified protection.
By default, allocation begins at the low end of the range. However, the
BOOTROM_ALLOC_TOP flag is provided to locate a provided bootrom in the high
region it is expected to be in.

The existing ROM-file loading code is refactored to use the new interface.

Reviewed by: grehan (earlier version)
Differential Revision: https://reviews.freebsd.org/D24422