Rather than just picking the first CD (or failing that, HDD) we come
across, if the user has picked a boot device ordering with <boot
order=''>, respect that (and just try to boot the lowest-index device).

Adds two sets of tests to bhyve2xmlargv; 'grub-bootorder' shows that we
pick a user-specified device over the first device in the domain;
'grub-bootorder2' shows that we pick the first (lowest index) device.

Update bhyveBuildDiskArgStr to support volumes:

 - Make virBhyveProcessBuildBhyveCmd and
   virBhyveProcessBuildLoadCmd take virConnectPtr as the
   first argument instead of bhyveConnPtr as virConnectPtr is
   needed for virStorageTranslateDiskSourcePool,
 - Add virStorageTranslateDiskSourcePool call to
   virBhyveProcessBuildBhyveCmd and
   virBhyveProcessBuildLoadCmd,
 - Allow disks of type VIR_STORAGE_TYPE_VOLUME

Add support for CDROM devices for bhyve driver using
bhyve(8)'s 'ahci-cd' device type.

As bhyve currently does not support media insertion at runtime,
disallow to start a domain with an empty source path for cdrom
devices.

* bhyve_command.c (bhyveBuildNetArgStr, virBhyveProcessBuildBhyveCmd):
  add dryRun mode which doesn't create any devices when enabled
* bhyve_command.c (virBhyveProcessBuildBhyveCmd,
  virBhyveProcessBuildDestroyCmd, virBhyveProcessBuildLoadCmd): accept
  virDomainDefPtr instead of virDomainObjPtr.

As we can use virBhyveCapsBuild() now, replace
testBhyveBuildCapabilities() with it.

At this point unittest covers 4 basic cases:

 - minimal working XML for bhyve
 - same as above, but with virtio disk
 - ACPI and APIC args test
 - MAC address test