qemu_conf.c: introduce qemuAddRemoveSharedDeviceInternal

After the previous commits, qemuAddSharedDevice() and
qemuRemoveSharedDevice() are now the same code with a different
flag to call the internal functions.

This patch aggregates the common code into a new function called
qemuAddRemoveSharedDeviceInternal() to further reduce
code repetition. Both qemuAddSharedDevice() and
qemuRemoveSharedDevice() are kept since they are public
functions used elsewhere.

No functional change was made.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>