virtPortProfile is now used by 4 different types of network devices
(NETWORK, BRIDGE, DIRECT, and HOSTDEV), and it's getting cumbersome to
replicate so much code in 4 different places just because each type
has the virtPortProfile in a slightly different place. This patch puts
a single virtPortProfile in a common place (outside the type-specific
union) in both virDomainNetDef and virDomainActualNetDef, and adjusts
the parse and format code (and the few other places where it is used)
accordingly.

Note that when a <virtualport> element is found, the parse functions
verify that the interface is of a type that supports one, otherwise an
error is generated (CONFIG_UNSUPPORTED in the case of <interface>, and
INTERNAL in the case of <actual>, since the contents of <actual> are
always generated by libvirt itself).