rbd: send mon_host for rbd drives

This is how the servers are passed to kvm.
libvirt: initial support for networked disks
rbd: support usernames, for cephx authentication
libvirt: extract usernames for network disks
rbd: Support using a custom secret
src/drives.c: 'auth' string is const.

This fixes commit f3e01b69e04a0e8321e9d54ca39a4d7ea762f80d.
src/libvirt-domain.c: Cast constant strings to avoid warning about signedness.
Use size_t for loop iterators.
rbd: Miscellaneous stylistic changes, no functional change.