Three new functions useful in other files:

virDomainHostdevInsert:

Add a new hostdev at the end of the array. This would more sensibly be
called virDomainHostdevAppend, but the existing functions for other
types of devices are called Insert.

virDomainHostdevRemove:

Eliminates one entry from the hostdevs array, but doesn't free it;
patterned after the code at the end of the two
qemuDomainDetachHostXXXDevice functions (and also other pre-existing
virDomainXXXRemove functions for other device types).

virDomainHostdevFind:

This function is patterned from the search loops at the top of
qemuDomainDetachHostPciDevice and qemuDomainDetachHostUsbDevice, and
will be used to re-factor those (and other detach-related) functions.