maint: prefer enum over int for virstoragefile structs

maint: prefer enum over int for virstoragefile structs

For internal structs, we might as well be type-safe and let the
compiler help us with less typing required on our part (getting
rid of casts is always nice).  In trying to use enums directly,
I noticed two problems in virstoragefile.h that can't be fixed
without more invasive refactoring: virStorageSource.format is
used as more of a union of multiple enums in storage volume
code (so it has to remain an int), and virStorageSourcePoolDef
refers to pooltype whose enum is declared in src/conf, but where
src/util can't pull in headers from src/conf.

* src/util/virstoragefile.h (virStorageNetHostDef)
(virStorageSourcePoolDef, virStorageSource): Use enums instead of
int for fields of internal types.
* src/qemu/qemu_command.c (qemuParseCommandLine): Cover all values.
* src/conf/domain_conf.c (virDomainDiskSourceParse)
(virDomainDiskSourceFormat): Simplify clients.
* src/qemu/qemu_driver.c
(qemuDomainSnapshotCreateSingleDiskActive)
(qemuDomainSnapshotPrepareDiskExternalBackingInactive)
(qemuDomainSnapshotPrepareDiskExternalOverlayActive)
(qemuDomainSnapshotPrepareDiskInternal): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>